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/sim_bridge.gd

@@ -396,6 +396,37 @@ func _test_snapshot() -> Dictionary:
 			"total_delay_ticks": total_delay,
 		})
 
+	# #535: Mock overheard NPC-NPC conversation (D-078)
+	# Two NPCs (Mira and Soren) trade lines every 5 ticks starting at tick 3.
+	# Conversation ends after 6 exchanges (~30 ticks).
+	var conv_events: Array = []
+	var conv_ended: Array = []
+	var conv_start := 3
+	var conv_lines := [
+		{"speaker": "Mira", "target": "Soren", "line": "The cargo manifests don't add up. Three containers unaccounted for."},
+		{"speaker": "Soren", "target": "Mira", "line": "Could be a logging error. Happens every... cycle."},
+		{"speaker": "Mira", "target": "Soren", "line": "Not like this. Someone moved them after... check."},
+		{"speaker": "Soren", "target": "Mira", "line": "You're reading too much into it. The docks are... these days."},
+		{"speaker": "Mira", "target": "Soren", "line": "Then explain the weight discrepancy. Two hundred kilos... just gone."},
+		{"speaker": "Soren", "target": "Mira", "line": "Fine. I'll pull the bay... tonight. But keep this between us."},
+	]
+	var conv_tick_interval := 5
+	var conv_total_ticks := conv_lines.size() * conv_tick_interval
+	if _test_tick >= conv_start and _test_tick < conv_start + conv_total_ticks:
+		var conv_index := (_test_tick - conv_start) / conv_tick_interval
+		var within_tick := (_test_tick - conv_start) % conv_tick_interval
+		if within_tick == 0 and conv_index < conv_lines.size():
+			var cl: Dictionary = conv_lines[conv_index]
+			conv_events.append({
+				"speaker_id": 10,
+				"target_id": 11,
+				"speaker_name": cl.speaker,
+				"target_name": cl.target,
+				"occluded_line": cl.line,
+			})
+	elif _test_tick == conv_start + conv_total_ticks:
+		conv_ended.append({"speaker_id": 10, "target_id": 11})
+
 	return {
 		"tick": _test_tick,
 		"version": Protocol.PROTOCOL_VERSION,
@@ -417,6 +448,8 @@ func _test_snapshot() -> Dictionary:
 		"current_dialogue": dialogue,
 		"pending_recognitions": pending_recs,
 		"gauntlet_mode": _test_gauntlet_mode,
+		"conversation_events": conv_events,
+		"conversation_ended": conv_ended,
 	}
 
 # Generate a small test room: 8x6 room with walls, a door, and floor