feat: include all group members in p-tags of inner ephemeral events

All signaling event types in group calls (answer, hangup, reject,
renegotiate) now include p-tags for every group member, matching the
pattern already used by CallOfferEvent. This allows signing the inner
event once and gift-wrapping it separately for each recipient.

- Add Set<HexKey> build() overloads to CallAnswerEvent, CallHangupEvent,
  CallRejectEvent, and CallRenegotiateEvent
- Add createGroupCallAnswer, createGroupRenegotiate factory methods
- Fix createGroupHangup to sign once instead of per-peer
- Update createGroupReject to accept full member set
- Update CallManager to use group methods when in group call
- Document group call p-tag convention in NIP-AC spec
- Add tests for all group build overloads

https://claude.ai/code/session_013h2E7spwHDgSjunqumsYgp

quartz/src/commonTest/kotlin/com/vitorpamplona/quartz/nipACWebRtcCalls/CallEventsTest.kt

@@ -249,6 +249,72 @@ class CallEventsTest {
         assertEquals((2000L + CallOfferEvent.EXPIRATION_SECONDS).toString(), expirationTag?.get(1))
     }
 
+    @Test
+    fun groupCallAnswerBuildIncludesAllPTags() {
+        val members = setOf("alice", "bob", "carol")
+        val template =
+            CallAnswerEvent.build(
+                sdpAnswer = "answer-sdp",
+                memberPubKeys = members,
+                callId = "group-call-1",
+            )
+        val pTagValues =
+            template.tags
+                .filter { it[0] == "p" }
+                .map { it[1] }
+                .toSet()
+        assertEquals(members, pTagValues)
+    }
+
+    @Test
+    fun groupCallHangupBuildIncludesAllPTags() {
+        val members = setOf("alice", "bob", "carol")
+        val template =
+            CallHangupEvent.build(
+                memberPubKeys = members,
+                callId = "group-call-1",
+            )
+        val pTagValues =
+            template.tags
+                .filter { it[0] == "p" }
+                .map { it[1] }
+                .toSet()
+        assertEquals(members, pTagValues)
+    }
+
+    @Test
+    fun groupCallRejectBuildIncludesAllPTags() {
+        val members = setOf("alice", "bob", "carol")
+        val template =
+            CallRejectEvent.build(
+                memberPubKeys = members,
+                callId = "group-call-1",
+            )
+        val pTagValues =
+            template.tags
+                .filter { it[0] == "p" }
+                .map { it[1] }
+                .toSet()
+        assertEquals(members, pTagValues)
+    }
+
+    @Test
+    fun groupCallRenegotiateBuildIncludesAllPTags() {
+        val members = setOf("alice", "bob", "carol")
+        val template =
+            CallRenegotiateEvent.build(
+                sdpOffer = "new-sdp-offer",
+                memberPubKeys = members,
+                callId = "group-call-1",
+            )
+        val pTagValues =
+            template.tags
+                .filter { it[0] == "p" }
+                .map { it[1] }
+                .toSet()
+        assertEquals(members, pTagValues)
+    }
+
     @Test
     fun singleCalleeOfferIsNotGroupCall() {
         val template =