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/main.gd

@@ -14,10 +14,12 @@ extends Node2D
 @onready var cursor_renderer = $UILayer/CursorRenderer  # D-056: z-layer 7
 @onready var gauntlet_hud = $UILayer/GauntletHUD  # #496: room timer + personal bests
 @onready var checklist_overlay = $UILayer/ChecklistOverlay  # #503: auto-checklist progress
+@onready var debug_overlay = $UILayer/DebugOverlay  # #511: F3 debug overlay
 @onready var bug_report_dialog = $ModalLayer/BugReportDialog  # #495: F12 WRONG button
 @onready var settings_dialog = $ModalLayer/SettingsDialog     # #528: audio settings (ESC/OPEN_MENU)
 
 var _last_dialogue_npc_id: int = -1  # D-064: NPC entity_id for WalkAway input
+var _last_dialogue_npc_name: String = ""  # #535: NPC name for dialogue_response attribution
 var _camera_anchored: bool = false
 var _last_monologue_tick: int = -1       # Prevent re-consuming monologue when same tick polled twice
 var _last_dialogue_tick: int = -1
@@ -130,6 +132,10 @@ func _process(_delta: float) -> void:
 		if checklist_overlay and checklist_overlay.has_method("update_from_state"):
 			checklist_overlay.update_from_state()
 
+		# #511: Update debug overlay (F3 toggle, dev tool)
+		if debug_overlay and debug_overlay.has_method("update_from_state"):
+			debug_overlay.update_from_state()
+
 		# D-018 #125: Play close-range sound events via positional 2D audio
 		_play_close_sound_events()
 
@@ -146,6 +152,11 @@ func _process(_delta: float) -> void:
 		# D-061: Show dialogue if server sent one this tick (#434)
 		_consume_dialogue()
 
+		# #535: Consume overheard conversation events and responses
+		_consume_conversation_events()
+		_consume_conversation_ended()
+		_consume_dialogue_response()
+
 	# Track camera to player position every frame (D-015: locked, no panning)
 	if _camera_anchored:
 		camera.global_position = GameState.player_position * Constants.TILE_SIZE
@@ -316,6 +327,7 @@ func _consume_dialogue() -> void:
 	_last_dialogue_tick = GameState.current_tick
 	var dlg: Dictionary = GameState.current_dialogue
 	_last_dialogue_npc_id = dlg.get("npc_entity_id", -1)
+	_last_dialogue_npc_name = dlg.get("npc_name", "")
 	dialogue_box.show_dialogue(
 		dlg.get("npc_name", ""),
 		dlg.get("speech", ""),
@@ -324,6 +336,34 @@ func _consume_dialogue() -> void:
 	GameState.current_dialogue = null
 
 
+# #535: Consume overheard NPC-NPC conversation events (D-078).
+# Each event carries pre-occluded text — render verbatim in the dialogue log.
+func _consume_conversation_events() -> void:
+	if not dialogue_box:
+		return
+	for event in GameState.conversation_events:
+		dialogue_box.append_conversation_event(event)
+	GameState.conversation_events = []
+
+
+# #535: Handle conversation_ended events — notify dialogue box to stop tracking pairs.
+func _consume_conversation_ended() -> void:
+	if not dialogue_box:
+		return
+	for event in GameState.conversation_ended:
+		dialogue_box.on_conversation_ended(event)
+	GameState.conversation_ended = []
+
+
+# #535: Consume dialogue_response — NPC follow-up line after player picks an option.
+func _consume_dialogue_response() -> void:
+	if GameState.dialogue_response == null or not dialogue_box:
+		return
+	var dr: Dictionary = GameState.dialogue_response
+	dialogue_box.append_dialogue_response(_last_dialogue_npc_name, dr.get("text", ""))
+	GameState.dialogue_response = null
+
+
 # D-061: Handle dialogue option selection → send to server
 func _on_dialogue_option_selected(response_id: String, text: String) -> void:
 	SimBridge.send_input({