feat(ui): unified dialogue log with overheard NPC conversations (#535)

Refactors dialogue box into a scrolling conversation log. All dialogue
(player-NPC and overheard NPC-NPC) flows chronologically, oldest at top.
Player response options at the bottom during active conversations.

- Entries expire after configurable timeout (equal for all message types)
- Walk-away clears options but preserves log entries (fair information)
- Per-character name colors from dialogue-theme.yaml (hash-indexed palette)
- Overheard lines render at 90% opacity (D-078)
- Protocol decode for conversation_events + conversation_ended
- GameState fields for conversation_events, conversation_ended, dialogue_response
- Mock Mira/Soren NPC-NPC conversation in test snapshot
- Also wires #511 debug overlay into main.gd and main.tscn

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

client/scripts/autoloads/game_state.gd

@@ -59,6 +59,11 @@ var rng_seed: Variant = null
 # v7 fields (#431, D-059/D-060)
 var pending_recognitions: Array = []  # [{entity_id, x, y, z, remaining_ticks, total_delay_ticks}]
 
+# v9 fields (#535, D-078): Overheard NPC-to-NPC conversations
+var conversation_events: Array = []  # [{speaker_id, target_id, speaker_name, target_name, occluded_line}]
+var conversation_ended: Array = []  # [{speaker_id, target_id}]
+var dialogue_response: Variant = null  # {line_id, text, speaker_entity_id} — NPC follow-up after player choice
+
 # #126, D-018: Medium-range sound events for fog-edge directional indicators.
 # Format: [{x, y, event_type, range_category}] — server sends current medium events per tick.
 var medium_sound_events: Array = []
@@ -169,6 +174,24 @@ func apply_snapshot(snapshot: Dictionary) -> void:
 	else:
 		pending_recognitions = []
 
+	# v9: conversation_events (#535, D-078) — overheard NPC-to-NPC lines
+	if snapshot.has("conversation_events") and snapshot.conversation_events is Array:
+		conversation_events = snapshot.conversation_events
+	else:
+		conversation_events = []
+
+	# v9: conversation_ended (#535, D-078) — pairs whose conversation ended
+	if snapshot.has("conversation_ended") and snapshot.conversation_ended is Array:
+		conversation_ended = snapshot.conversation_ended
+	else:
+		conversation_ended = []
+
+	# v9: dialogue_response (#535, D-028) — NPC follow-up after player choice
+	if snapshot.has("dialogue_response") and snapshot.dialogue_response is Dictionary:
+		dialogue_response = snapshot.dialogue_response
+	else:
+		dialogue_response = null
+
 	# v8: gauntlet mode (#496) — room_id and gauntlet_mode
 	if snapshot.has("gauntlet_mode") and snapshot.gauntlet_mode == true:
 		gauntlet_mode = true