feat(ui): add in-game debug console with tilde toggle and command dispatch (#581)

- debug_console.gd: new ModalLayer Control — tilde key toggles bottom-40% panel,
  command history (up/down), SimBridge dispatch for all DebugCommandKind variants:
  ticks, contaminate, tp, activate, triangle, npc, triangles, pop, status, help
- debug_console.tscn: minimal scene node; UI built programmatically in _ready()
- input_mapper.gd: DEBUG_COMMAND action added to Action enum
- sim_bridge.gd: DEBUG_COMMAND → "DebugCommand" wire mapping
- protocol.gd: v18 debug_response decode (command, text, success fields)
- game_state.gd: debug_response field + apply_snapshot one-shot handling
- main.gd: @onready ref, router registration, _consume_debug_response(), settings signal
- settings_dialog.gd: debug_console_toggled signal + CheckButton toggle row (+36px height),
  reads initial state from user://settings.cfg; CheckButton state loaded from PREFS_PATH
- main.tscn: DebugConsole node on ModalLayer, load_steps 27→28

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

client/scripts/protocol/protocol.gd

@@ -244,6 +244,17 @@ static func decode_snapshot(bytes: PackedByteArray) -> Variant:
 			"error": raw_save.get("error"),
 		}
 
+	# v18: debug_response (#580) — debug console command result.
+	# {command: String, text: String, success: bool}
+	var debug_response: Variant = null
+	var raw_debug: Variant = raw.get("debug_response")
+	if raw_debug is Dictionary:
+		debug_response = {
+			"command": str(raw_debug.get("command", "")),
+			"text": str(raw_debug.get("text", "")),
+			"success": bool(raw_debug.get("success", false)),
+		}
+
 	# TODO(server): Send stationary_ticks in ObserverSnapshot (D-071, D-020).
 	# Server already tracks this in ListeningFocus component (server/src/simulation/listening.rs).
 	# When server populates this field, client-side accumulation fallback in game_state.gd
@@ -320,6 +331,7 @@ static func decode_snapshot(bytes: PackedByteArray) -> Variant:
 		"examine_result": examine_result,
 		"player_knowledge": player_knowledge,
 		"save_result": save_result,
+		"debug_response": debug_response,
 		"stationary_ticks": stationary_ticks,
 		"zone_id": zone_id,
 	}