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/protocol/protocol.gd

@@ -180,6 +180,32 @@ static func decode_snapshot(bytes: PackedByteArray) -> Variant:
 			"speaker_entity_id": int(raw_dr.get("speaker_entity_id", -1)),
 		}
 
+	# v9: conversation_events (#535, D-078) — overheard NPC-to-NPC dialogue lines.
+	# Each event carries pre-occluded text plus speaker/target attribution.
+	var conversation_events: Array = []
+	var raw_conv_events: Variant = raw.get("conversation_events")
+	if raw_conv_events is Array:
+		for raw_ce in raw_conv_events:
+			if raw_ce is Dictionary and raw_ce.has("occluded_line"):
+				conversation_events.append({
+					"speaker_id": int(raw_ce.get("speaker_id", 0)),
+					"target_id": int(raw_ce.get("target_id", 0)),
+					"speaker_name": str(raw_ce.get("speaker_name", "")),
+					"target_name": str(raw_ce.get("target_name", "")),
+					"occluded_line": str(raw_ce["occluded_line"]),
+				})
+
+	# v9: conversation_ended (#535, D-078) — pairs whose conversation ended this tick.
+	var conversation_ended: Array = []
+	var raw_conv_ended: Variant = raw.get("conversation_ended")
+	if raw_conv_ended is Array:
+		for raw_end in raw_conv_ended:
+			if raw_end is Dictionary:
+				conversation_ended.append({
+					"speaker_id": int(raw_end.get("speaker_id", 0)),
+					"target_id": int(raw_end.get("target_id", 0)),
+				})
+
 	return {
 		"tick": tick,
 		"entities": entities,
@@ -195,6 +221,8 @@ static func decode_snapshot(bytes: PackedByteArray) -> Variant:
 		"current_dialogue": current_dialogue,
 		"dialogue_response": dialogue_response,
 		"pending_recognitions": pending_recognitions,
+		"conversation_events": conversation_events,
+		"conversation_ended": conversation_ended,
 	}