Add session shell and close world presentation follow-ups

Session shell
- Startup venue menu (src/game/ui/start_menu.gd): keyboard + mouse, replaces
  the live-shift boot and V as the player-facing venue path (V stays a debug bind).
- Pause overlay on Esc (src/game/ui/pause_overlay.gd): resume, restart shift,
  settings, quit to menu.
- Settings panel (src/game/ui/settings_panel.gd): mouse look sensitivity and
  HUD font scale, persisted via Save into user://settings.json and loaded on boot.
- Shift review "Run another shift" now returns to the venue menu.
- New smoke: src/test/smoke_session_shell.gd (menu -> shift -> pause -> settings
  -> menu), passes headless and windowed.

World and presentation
- Skyline backdrop restored with a transparent asset and an alpha-scissor
  unshaded material; fixes the opaque black slab from the earlier attempt and
  the unimported texture path.
- Steps and landing are a StaticBody3D with per-tread colliders, so the guard
  and NPCs walk up instead of clipping through.
- Marquee Label3D scaled up (72 -> 160) so STEP LINE reads from the sidewalk.
- Focus torus and name tag enlarged for 2-3 m readability.
- Routine traffic despawns after an idle lifetime (NpcAvatar.ephemeral), so the
  16-NPC cap no longer fills with standers; authored events stay for the shift.

Docs and tests
- PLAN.md test/CI section corrected to the custom headless runner and Gitea Actions.
- README controls table, smoke checklist, test count, and status lists refreshed.
- TODO/DONE/MEMORY updated; completed sections moved to DONE.md.
- Headless suite 77 -> 88 passing (settings round-trip, step colliders, skyline
  alpha, session-shell UI behavior, traffic lifetime).

src/test/tests/test_persistence.gd

@@ -30,3 +30,24 @@ func test_load_returns_array_when_empty_or_missing() -> bool:
 	var ok := TestUtils.check(results is Array, "load_shift_results always returns Array")
 	save.free()
 	return ok
+
+func test_settings_round_trip() -> bool:
+	var save := _save_instance()
+	var err: int = save.save_settings({"look_sensitivity": 0.0035, "hud_font_scale": 1.25})
+	var a := TestUtils.check_eq(err, OK, "save_settings succeeds")
+	var loaded: Dictionary = save.load_settings()
+	var b := TestUtils.check(is_equal_approx(float(loaded.get("look_sensitivity", 0.0)), 0.0035), "round-trips sensitivity")
+	var c := TestUtils.check(is_equal_approx(float(loaded.get("hud_font_scale", 0.0)), 1.25), "round-trips font scale")
+	# Restore defaults so later runs are not affected.
+	save.save_settings(save.DEFAULT_SETTINGS)
+	save.free()
+	return a and b and c
+
+func test_settings_defaults_present() -> bool:
+	var save := _save_instance()
+	var defaults: Dictionary = save.load_settings()
+	var ok := TestUtils.check(
+		defaults.has("look_sensitivity") and defaults.has("hud_font_scale"),
+		"settings have documented defaults")
+	save.free()
+	return ok