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/npc_avatar.gd

@@ -12,6 +12,11 @@ var target_position: Vector3 = Vector3.ZERO
 var door_position: Vector3 = Vector3(0, 0.5, 1.2)
 var awaiting_interaction := false
 var arrived := false
+## Seconds idle at an arrived waypoint. Ordinary traffic lives for a while, then
+## despawns so the entrance does not fill with standers (TODO: NPC crowd cap).
+var idle_s := 0.0
+var lifetime_s := 90.0
+var ephemeral := true
 var agent: NavigationAgent3D = null
 ## Set once the nav agent proves it has a path; direct movement is the fallback.
 var _nav_usable := false
@@ -123,8 +128,8 @@ func _build_focus_ring() -> void:
 	_ring = MeshInstance3D.new()
 	_ring.name = "FocusRing"
 	var torus := TorusMesh.new()
-	torus.inner_radius = 0.16
-	torus.outer_radius = 0.20
+	torus.inner_radius = 0.26
+	torus.outer_radius = 0.34
 	torus.rings = 12
 	torus.ring_segments = 16
 	_ring.mesh = torus
@@ -139,10 +144,10 @@ func _build_focus_ring() -> void:
 	_name_tag = Label3D.new()
 	_name_tag.name = "NameTag"
 	_name_tag.text = npc.profile.display_name if npc and npc.profile else ""
-	_name_tag.font_size = 28
+	_name_tag.font_size = 48
 	_name_tag.pixel_size = 0.004
 	_name_tag.billboard = BaseMaterial3D.BILLBOARD_ENABLED
-	_name_tag.position = Vector3(0, 0.42, 0)
+	_name_tag.position = Vector3(0, 0.58, 0)
 	_name_tag.modulate = Color(0.95, 0.95, 0.9, 0.9)
 	_name_tag.visible = false
 	_head.add_child(_name_tag)
@@ -257,20 +262,31 @@ func _animate(delta: float) -> void:
 			_head.rotation.y = lerp_angle(_head.rotation.y, world_yaw - parent_yaw, 0.1)
 
 func _advance_fsm(delta: float) -> void:
+	# Routine traffic despawns after a while once it has finished its walk, so
+	# the crowd cap is not permanently occupied by idle standers.
+	if ephemeral and arrived and npc.fsm.state != "LEAVING":
+		idle_s += delta
+		if idle_s >= lifetime_s:
+			leave_requested_by_lifetime = true
+			return
 	# Returning-trespasser: an NPC directed away may circle back to the door.
 	if npc.tick_return(delta):
 		arrived = false
 		awaiting_interaction = false
 		target_position = door_position
 		return
-	if npc.leave_requested:
+	if npc.leave_requested or leave_requested_by_lifetime:
 		var away := global_position + (global_position - spawn_position).normalized() * 40.0
 		away.y = 0.5
 		target_position = away
+		if (global_position - away).length() < 2.0:
+			queue_free()
 		return
 	if arrived and not awaiting_interaction:
 		awaiting_interaction = true
 
+var leave_requested_by_lifetime := false
+
 func _move_toward_target(delta: float) -> void:
 	if agent == null:
 		_move_direct(delta)