feat(audio-rooms): LazyHorizontalGrid participant renderer (T2 #1)

Replace the two LazyRow stage/audience sections with a single
ParticipantsGrid composable backed by buildParticipantGrid in
commons. The grid:

  * derives on-stage from kind-30312 role + kind-10312 onstage flag
  * renders absent members (promoted but never emitted presence) at
    50 % alpha — matches nostrnests' grey-out for "promoted but
    never joined"
  * uses LazyHorizontalGrid with a 1-row Fixed cell so adding a
    second row (e.g. names below avatars) is a one-line change

Audience derivation moves from AudioRoomActivityContent into the
projection function — the activity-side filter for "neither host
nor speaker" was only used by the now-removed StagePeopleRow call,
so deleting it shrinks the activity composable and keeps both
audience and absence logic in commons.

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/audiorooms/room/AudioRoomActivityContent.kt

@@ -117,13 +117,10 @@ private fun AudioRoomActivityBody(
     val participants = remember(event) { event.participants() }
     val hosts = remember(participants) { participants.filter { it.role.equals(ROLE.HOST.code, true) } }
     val speakers = remember(participants) { participants.filter { it.role.equals(ROLE.SPEAKER.code, true) } }
-    val audience =
+    // The grid renderer derives audience from `event.participants()` +
-        remember(participants) {
+    // the kind-10312 presence aggregator (see ParticipantsGrid /
-            participants.filter {
+    // buildParticipantGrid). This composable only needs the on-stage
-                !it.role.equals(ROLE.HOST.code, true) &&
+    // subset for the talk-row gate + PIP renderer.
-                    !it.role.equals(ROLE.SPEAKER.code, true)
-            }
-        }
     val onStage = remember(hosts, speakers) { hosts + speakers }
     val onStageKeys = remember(onStage) { onStage.map { it.pubKey }.toSet() }
 
@@ -350,7 +347,6 @@ private fun AudioRoomActivityBody(
             event = event,
             roomNote = roomNote,
             onStage = onStage,
-            audience = audience,
             viewModel = viewModel,
             ui = ui,
             accountViewModel = accountViewModel,

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/audiorooms/room/AudioRoomFullScreen.kt

@@ -61,8 +61,6 @@ import com.vitorpamplona.amethyst.commons.viewmodels.ConnectionUiState
 import com.vitorpamplona.amethyst.commons.viewmodels.RoomTheme
 import com.vitorpamplona.amethyst.ui.screen.loggedIn.AccountViewModel
 import com.vitorpamplona.amethyst.ui.stringRes
-import com.vitorpamplona.amethyst.ui.theme.Size35dp
-import com.vitorpamplona.amethyst.ui.theme.Size40dp
 import com.vitorpamplona.quartz.nip01Core.tags.aTag.ATag
 import com.vitorpamplona.quartz.nip19Bech32.toNAddr
 import com.vitorpamplona.quartz.nip53LiveActivities.meetingSpaces.MeetingSpaceEvent
@@ -82,7 +80,6 @@ internal fun AudioRoomFullScreen(
     event: MeetingSpaceEvent,
     roomNote: com.vitorpamplona.amethyst.model.AddressableNote,
     onStage: List<ParticipantTag>,
-    audience: List<ParticipantTag>,
     viewModel: AudioRoomViewModel,
     ui: AudioRoomUiState,
     accountViewModel: AccountViewModel,
@@ -191,27 +188,26 @@ internal fun AudioRoomFullScreen(
             val onLongPressParticipant: ((String) -> Unit) = { target ->
                 if (target != accountViewModel.account.signer.pubKey) hostMenuTarget = target
             }
-            if (onStage.isNotEmpty()) {
+            // Tier-2 #1: replace the two LazyRow sections with a single
-                StagePeopleRow(
+            // pure-projection ParticipantGrid. The `absent` flag (member
-                    label = stringRes(R.string.audio_room_stage),
+            // promoted in the kind-30312 but never emitted a kind-10312)
-                    people = onStage,
+            // greys out at 50 % alpha, matching nostrnests' web client.
-                    avatarSize = Size40dp,
+            val participantGrid =
-                    speakingNow = ui.speakingNow,
+                androidx.compose.runtime.remember(event, presences) {
-                    accountViewModel = accountViewModel,
+                    com.vitorpamplona.amethyst.commons.viewmodels.buildParticipantGrid(
-                    reactionsByPubkey = reactionsByPubkey,
+                        participants = event.participants(),
-                    onLongPressParticipant = onLongPressParticipant,
+                        presences = presences,
-                )
+                    )
-            }
+                }
-            if (audience.isNotEmpty()) {
+            ParticipantsGrid(
-                StagePeopleRow(
+                grid = participantGrid,
-                    label = stringRes(R.string.audio_room_audience),
+                speakingNow = ui.speakingNow,
-                    people = audience,
+                accountViewModel = accountViewModel,
-                    avatarSize = Size35dp,
+                onStageLabel = stringRes(R.string.audio_room_stage),
-                    speakingNow = kotlinx.collections.immutable.persistentSetOf(),
+                audienceLabel = stringRes(R.string.audio_room_audience),
-                    accountViewModel = accountViewModel,
+                reactionsByPubkey = reactionsByPubkey,
-                    onLongPressParticipant = onLongPressParticipant,
+                onLongPressParticipant = onLongPressParticipant,
-                )
+            )
-            }
             hostMenuTarget?.let { target ->
                 ParticipantHostActionsSheet(
                     target = target,

amethyst/src/main/java/com/vitorpamplona/amethyst/ui/screen/loggedIn/audiorooms/room/ParticipantsGrid.kt

@@ -0,0 +1,162 @@
+/*
+ * Copyright (c) 2025 Vitor Pamplona
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy of
+ * this software and associated documentation files (the "Software"), to deal in
+ * the Software without restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the
+ * Software, and to permit persons to whom the Software is furnished to do so,
+ * subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS
+ * FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR
+ * COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN
+ * AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
+ * WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
+ */
+package com.vitorpamplona.amethyst.ui.screen.loggedIn.audiorooms.room
+
+import androidx.compose.foundation.border
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.Spacer
+import androidx.compose.foundation.layout.fillMaxWidth
+import androidx.compose.foundation.layout.height
+import androidx.compose.foundation.layout.padding
+import androidx.compose.foundation.lazy.grid.GridCells
+import androidx.compose.foundation.lazy.grid.LazyHorizontalGrid
+import androidx.compose.foundation.lazy.grid.items
+import androidx.compose.foundation.shape.CircleShape
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.draw.alpha
+import androidx.compose.ui.unit.dp
+import com.vitorpamplona.amethyst.commons.viewmodels.ParticipantGrid
+import com.vitorpamplona.amethyst.commons.viewmodels.RoomMember
+import com.vitorpamplona.amethyst.commons.viewmodels.RoomReaction
+import com.vitorpamplona.amethyst.ui.note.ClickableUserPicture
+import com.vitorpamplona.amethyst.ui.screen.loggedIn.AccountViewModel
+import com.vitorpamplona.amethyst.ui.theme.Size35dp
+import com.vitorpamplona.amethyst.ui.theme.Size40dp
+import kotlinx.collections.immutable.ImmutableSet
+
+/**
+ * Material3 grid for the room participants — replaces the
+ * horizontal LazyRow layout for rooms with many speakers /
+ * audience members. On-stage and audience render as separate
+ * fixed-height horizontal grids (one row per group, scrolls
+ * horizontally) so the chat panel below stays at a predictable
+ * vertical position even when the room fills up.
+ *
+ * Falls open: when [grid] is empty (room with no presence yet),
+ * neither section renders.
+ *
+ * Absent members (kind-30312 `p`-tag with no kind-10312 presence)
+ * render at 50% alpha — matches nostrnests' grey-out for
+ * "promoted but never joined".
+ */
+@Composable
+internal fun ParticipantsGrid(
+    grid: ParticipantGrid,
+    speakingNow: ImmutableSet<String>,
+    accountViewModel: AccountViewModel,
+    onStageLabel: String,
+    audienceLabel: String,
+    modifier: Modifier = Modifier,
+    reactionsByPubkey: Map<String, List<RoomReaction>> = emptyMap(),
+    onLongPressParticipant: ((String) -> Unit)? = null,
+) {
+    Column(modifier = modifier.fillMaxWidth()) {
+        if (grid.onStage.isNotEmpty()) {
+            ParticipantsSection(
+                title = onStageLabel,
+                members = grid.onStage,
+                avatarSize = Size40dp,
+                speakingNow = speakingNow,
+                accountViewModel = accountViewModel,
+                reactionsByPubkey = reactionsByPubkey,
+                onLongPressParticipant = onLongPressParticipant,
+            )
+        }
+        if (grid.audience.isNotEmpty()) {
+            Spacer(Modifier.height(8.dp))
+            ParticipantsSection(
+                title = audienceLabel,
+                members = grid.audience,
+                avatarSize = Size35dp,
+                // Audience rows don't get the speaking-ring — only
+                // members with a live broadcast track do. Pass an
+                // empty set rather than thread a per-section bool.
+                speakingNow = kotlinx.collections.immutable.persistentSetOf(),
+                accountViewModel = accountViewModel,
+                reactionsByPubkey = emptyMap(),
+                onLongPressParticipant = onLongPressParticipant,
+            )
+        }
+    }
+}
+
+@Composable
+private fun ParticipantsSection(
+    title: String,
+    members: List<RoomMember>,
+    avatarSize: androidx.compose.ui.unit.Dp,
+    speakingNow: ImmutableSet<String>,
+    accountViewModel: AccountViewModel,
+    reactionsByPubkey: Map<String, List<RoomReaction>>,
+    onLongPressParticipant: ((String) -> Unit)?,
+) {
+    val ringColor = MaterialTheme.colorScheme.primary
+    Column(modifier = Modifier.padding(top = 8.dp)) {
+        Text(
+            text = title,
+            style = MaterialTheme.typography.labelMedium,
+            color = MaterialTheme.colorScheme.onSurfaceVariant,
+        )
+        // Single-row horizontal grid — visually identical to the
+        // existing LazyRow layout but uses LazyHorizontalGrid so
+        // adding a second row (e.g. names below avatars) is a
+        // one-line change later. Cell size matches avatarSize +
+        // padding for the reaction overlay below.
+        LazyHorizontalGrid(
+            rows = GridCells.Fixed(1),
+            modifier =
+                Modifier
+                    .fillMaxWidth()
+                    .height(avatarSize + 32.dp)
+                    .padding(top = 4.dp),
+            horizontalArrangement = Arrangement.spacedBy(6.dp),
+        ) {
+            items(items = members, key = { it.pubkey }) { member ->
+                val isSpeaking = member.pubkey in speakingNow
+                val avatarModifier =
+                    Modifier
+                        .let { if (isSpeaking) it.border(2.dp, ringColor, CircleShape) else it }
+                        .let { if (member.absent) it.alpha(0.5f) else it }
+                Column(horizontalAlignment = Alignment.CenterHorizontally) {
+                    ClickableUserPicture(
+                        baseUserHex = member.pubkey,
+                        size = avatarSize,
+                        accountViewModel = accountViewModel,
+                        modifier = avatarModifier,
+                        onLongClick = onLongPressParticipant?.let { cb -> { hex -> cb(hex) } },
+                    )
+                    val reactions = reactionsByPubkey[member.pubkey].orEmpty()
+                    if (reactions.isNotEmpty()) {
+                        SpeakerReactionOverlay(
+                            reactions = reactions,
+                            modifier = Modifier.padding(top = 2.dp),
+                        )
+                    }
+                }
+            }
+        }
+    }
+}