feat(ui): add diegetic time display on insert HUD (#263)

Time display on InsertOverlay (CanvasLayer 10) shows station local
time (HH:MM), day phase with cycle-tinted color, and day number.
Reads SimulationTime from GameState.game_time via update_from_state().
Adds Constants.format_game_time() helper for testability.
Placeholder layout — position refines when #314 wireframe lands.

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

client/scripts/constants.gd

@@ -96,6 +96,12 @@ const FACING_INDICATOR_OFFSET: float = 14.0
 # two columns of text comfortably, leaves world game visible alongside.
 const DIALOGUE_MAX_WIDTH: int = 1200
 
+# D-031: Format game-minutes (0..1439) as station local time string "HH:MM".
+static func format_game_time(time_of_day: int) -> String:
+	var hours: int = time_of_day / 60
+	var minutes: int = time_of_day % 60
+	return "%02d:%02d" % [hours, minutes]
+
 # Default camera zoom — used as fallback when get_camera_2d() returns null
 const CAMERA_DEFAULT_ZOOM: Vector2 = Vector2(2.0, 2.0)