feat(audio-rooms): font-tag parser + system-font typography (T3 #1)

Closes the deferred font tag from the Tier-3 plan. Wire-up:

  * quartz: FontTag(family, optionalUrl) parser/assembler with
    blank-rejection on family + blank-URL-becomes-null normalisation.
  * MeetingSpaceEvent.font(): FontTag? accessor.
  * TagArrayBuilder.font(family, url) DSL.
  * RoomTheme gains fontFamily / fontUrl fields.
  * AudioRoomThemedScope maps `family` to a Compose FontFamily for
    the four CSS-style generic-family names ("sans-serif", "serif",
    "monospace", "cursive") — each maps to the corresponding system
    fallback. The whole Material3 typography is rebuilt with that
    family so headlines / body / labels all swap together.

URL-based font loading (RoomTheme.fontUrl) is the natural follow-up:
fetch + cache via OkHttp, then build a FontFamily from a local
file. Until then, an unknown family is silently a no-op — the room
renders in the platform default rather than crashing or fetching
on the UI thread.

Tests:
  * FontTagTest — 9 cases covering family-only, family+URL,
    blank rejection, missing family, wrong tag name, blank-URL
    normalisation, assembler shapes (with + without URL), and
    the assembler's blank-family rejection.
  * RoomThemeTest gains 3 cases for font projection (family-only /
    family+URL / empty event).

commons/src/commonMain/kotlin/com/vitorpamplona/amethyst/commons/viewmodels/RoomTheme.kt

@@ -43,11 +43,35 @@ data class RoomTheme(
     val primaryArgb: Long?,
     val backgroundImageUrl: String?,
     val backgroundMode: BackgroundMode,
+    /**
+     * Suggested font family from the kind-30312 `["f", family,
+     * optionalUrl]` tag. Renderers map this to a platform
+     * FontFamily — the system-font shorthand list ("sans-serif",
+     * "serif", "monospace", "cursive") is honoured directly;
+     * other names fall back to the platform default unless the
+     * renderer also fetches [fontUrl].
+     */
+    val fontFamily: String?,
+    /**
+     * Optional URL to a font file (e.g. WOFF2). The protocol
+     * permits async loading; clients without a font-fetch loader
+     * stay on [fontFamily] alone (or default typography on miss).
+     */
+    val fontUrl: String?,
 ) {
     enum class BackgroundMode { COVER, TILE }
 
     companion object {
-        val Empty = RoomTheme(null, null, null, null, BackgroundMode.COVER)
+        val Empty =
+            RoomTheme(
+                backgroundArgb = null,
+                textArgb = null,
+                primaryArgb = null,
+                backgroundImageUrl = null,
+                backgroundMode = BackgroundMode.COVER,
+                fontFamily = null,
+                fontUrl = null,
+            )
 
         /**
          * Project the theme tags off a kind-30312 event. Returns
@@ -57,6 +81,7 @@ data class RoomTheme(
         fun from(event: MeetingSpaceEvent): RoomTheme {
             val colors = event.colors()
             val bg = event.background()
+            val font = event.font()
 
             fun pickHex(target: ColorTag.Target): Long? =
                 colors
@@ -74,6 +99,8 @@ data class RoomTheme(
                         BackgroundTag.Mode.TILE -> BackgroundMode.TILE
                         else -> BackgroundMode.COVER
                     },
+                fontFamily = font?.family,
+                fontUrl = font?.url,
             )
         }
 

commons/src/commonTest/kotlin/com/vitorpamplona/amethyst/commons/viewmodels/RoomThemeTest.kt

@@ -128,4 +128,28 @@ class RoomThemeTest {
         assertEquals(0xFF000000L, RoomTheme.hexToOpaqueArgb("000000"))
         assertEquals(0xFFFFFFFFL, RoomTheme.hexToOpaqueArgb("FFFFFF"))
     }
+
+    @Test
+    fun fontFamilyOnly() {
+        val theme = RoomTheme.from(event(listOf(arrayOf("f", "Inter"))))
+        assertEquals("Inter", theme.fontFamily)
+        assertNull(theme.fontUrl)
+    }
+
+    @Test
+    fun fontFamilyAndUrl() {
+        val theme =
+            RoomTheme.from(
+                event(listOf(arrayOf("f", "Inter", "https://fonts.example/inter.woff2"))),
+            )
+        assertEquals("Inter", theme.fontFamily)
+        assertEquals("https://fonts.example/inter.woff2", theme.fontUrl)
+    }
+
+    @Test
+    fun emptyFontDefaultsBothNull() {
+        val theme = RoomTheme.from(event(emptyList()))
+        assertNull(theme.fontFamily)
+        assertNull(theme.fontUrl)
+    }
 }