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/scenes/main.tscn

@@ -1,4 +1,4 @@
-[gd_scene load_steps=23 format=3 uid="uid://bswrmh7w8dbgm"]
+[gd_scene load_steps=24 format=3 uid="uid://bswrmh7w8dbgm"]
 
 [ext_resource type="Script" path="res://scripts/main.gd" id="1_main"]
 [ext_resource type="Script" path="res://scripts/rendering/world_renderer.gd" id="2_world"]
@@ -22,6 +22,7 @@
 [ext_resource type="PackedScene" path="res://ui/bug_report_dialog.tscn" id="19_bugreport"]
 [ext_resource type="PackedScene" path="res://ui/settings_dialog.tscn" id="21_settings"]
 [ext_resource type="Script" path="res://scripts/ui/debug_overlay.gd" id="22_debug"]
+[ext_resource type="PackedScene" path="res://ui/time_display.tscn" id="23_tdisplay"]
 
 [node name="Game" type="Node2D"]
 script = ExtResource("1_main")
@@ -118,6 +119,9 @@ zoom = Vector2(2, 2)
 [node name="InsertOverlay" type="CanvasLayer" parent="."]
 layer = 10
 
+; #263: Time display — diegetic insert clock, top-left placeholder (D-013, D-031)
+[node name="TimeDisplay" parent="InsertOverlay" instance=ExtResource("23_tdisplay")]
+
 ; InteractionPrompt — v0.1 fallback single-line "E - Talk" display
 [node name="InteractionPrompt" parent="InsertOverlay" instance=ExtResource("9_prompt")]