feat(simulation): NPC-to-NPC conversation system (#247, D-078)

Implements server-authoritative per-word occlusion for NPC conversations.
ConversationEventBuffer drains into ObserverSnapshot each tick so the client
receives only words audible from the player's position. Updates test fixtures
to include the new conversation_events and conversation_ended fields.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/bridge/text_renderer.rs

@@ -300,6 +300,8 @@ mod tests {
             dialogue_response: None,
             blocked_entities: vec![],
             scan_events: vec![],
+            conversation_events: vec![],
+            conversation_ended: vec![],
             sound_events: vec![],
             rng_seed: None,
         }
@@ -425,6 +427,8 @@ mod tests {
             dialogue_response: None,
             blocked_entities: vec![],
             scan_events: vec![],
+            conversation_events: vec![],
+            conversation_ended: vec![],
             sound_events: vec![],
             rng_seed: None,
         };

server/src/bridge/types.rs

@@ -15,7 +15,7 @@ pub use crate::simulation::time::{DayPhase, TickRate};
 /// negotiation is unnecessary. Client should reject snapshots with version !=
 /// PROTOCOL_VERSION. New fields use #[serde(default)] only during the migration
 /// period, then the default is removed once both sides are updated.
-pub const PROTOCOL_VERSION: u8 = 11;
+pub const PROTOCOL_VERSION: u8 = 12;
 
 /// The ONLY data structure crossing the client-server boundary (D-020)
 /// Contains all information visible to the observer at a given tick.
@@ -31,10 +31,11 @@ pub const PROTOCOL_VERSION: u8 = 11;
 /// v10 adds: sound_events (#124, D-038 server sound event pipeline),
 ///           rng_seed (#527, deterministic replay — completes WRONG button loop).
 /// v11 adds: zone_id on VisibleTile (#523, D-077 OQ-09 resolution + D-073 crossfade).
+/// v12 adds: conversation_events, conversation_ended (#247, D-078 NPC-to-NPC conversations).
 /// Future fields: ambient sound events, HUD state (D-020 expansion).
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct ObserverSnapshot {
-    /// Protocol version for forward compatibility. Current: 11.
+    /// Protocol version for forward compatibility. Current: 12.
     pub version: u8,
     /// Simulation tick when this snapshot was produced
     pub tick: u64,
@@ -88,6 +89,15 @@ pub struct ObserverSnapshot {
     /// Empty when no sounds are in range.
     #[serde(default)]
     pub sound_events: Vec<crate::simulation::sound::SoundEvent>,
+    /// Overheard NPC-to-NPC conversation lines this tick (#247, D-078).
+    /// Each event carries pre-occluded text — client renders verbatim.
+    /// Empty when no conversations are overheard.
+    #[serde(default)]
+    pub conversation_events: Vec<crate::simulation::conversation::ConversationEvent>,
+    /// Conversations that ended this tick (#247, D-078).
+    /// Client dismisses the passive dialogue panel for these pairs.
+    #[serde(default)]
+    pub conversation_ended: Vec<crate::simulation::conversation::ConversationEndEvent>,
     /// RNG seed active at this tick for deterministic replay (#527).
     /// The WRONG button writes this to seed.txt so replays reproduce observed bugs.
     /// None when the RNG resource is unavailable (should not occur in practice).