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/game/ui/hud.gd

@@ -279,6 +279,18 @@ func set_wand_icon(visible: bool) -> void:
 	if wand_icon:
 		wand_icon.visible = visible
 
+## Scale the whole HUD text block (accessibility settings).
+func set_font_scale(scale: float) -> void:
+	var clamped := clampf(scale, 0.75, 1.75)
+	if top_label:
+		top_label.scale = Vector2(clamped, clamped)
+	if log_label:
+		log_label.scale = Vector2(clamped, clamped)
+	if target_label:
+		target_label.scale = Vector2(clamped, clamped)
+	if hint_label:
+		hint_label.scale = Vector2(clamped, clamped)
+
 func _make_icon(texture: Texture2D) -> TextureRect:
 	var tr := TextureRect.new()
 	tr.texture = texture