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/pause_overlay.gd

@@ -0,0 +1,84 @@
+class_name PauseOverlay
+extends CanvasLayer
+## Shift pause overlay: resume, restart the shift, open settings, or quit to the
+## venue menu. Opened with Esc from the world (or a dedicated pause action) and
+## while the overlay is open the guard is frozen.
+
+signal resume_requested()
+signal restart_requested()
+signal settings_requested()
+signal quit_to_menu_requested()
+
+var _root: Control = null
+var _resume_button: Button = null
+
+func _ready() -> void:
+	layer = 18
+	_build()
+	hide()
+
+func _build() -> void:
+	_root = Control.new()
+	_root.name = "PauseRoot"
+	_root.set_anchors_preset(Control.PRESET_FULL_RECT)
+	_root.mouse_filter = Control.MOUSE_FILTER_STOP
+	add_child(_root)
+
+	var bg := ColorRect.new()
+	bg.color = Color(0.02, 0.03, 0.05, 0.82)
+	bg.set_anchors_preset(Control.PRESET_FULL_RECT)
+	bg.mouse_filter = Control.MOUSE_FILTER_IGNORE
+	_root.add_child(bg)
+
+	var center := CenterContainer.new()
+	center.set_anchors_preset(Control.PRESET_FULL_RECT)
+	_root.add_child(center)
+
+	var column := VBoxContainer.new()
+	column.custom_minimum_size = Vector2(320, 0)
+	column.add_theme_constant_override("separation", 10)
+	center.add_child(column)
+
+	var title := Label.new()
+	title.text = "Paused"
+	title.add_theme_font_size_override("font_size", 36)
+	title.horizontal_alignment = HORIZONTAL_ALIGNMENT_CENTER
+	column.add_child(title)
+
+	_resume_button = _add_button(column, "Resume", "ResumeButton", func(): resume_requested.emit())
+	_add_button(column, "Restart shift", "RestartButton", func(): restart_requested.emit())
+	_add_button(column, "Settings", "SettingsButton", func(): settings_requested.emit())
+	_add_button(column, "Quit to menu", "MenuButton", func(): quit_to_menu_requested.emit())
+
+	var hint := Label.new()
+	hint.text = "Esc resumes"
+	hint.horizontal_alignment = HORIZONTAL_ALIGNMENT_CENTER
+	hint.add_theme_color_override("font_color", Color(0.55, 0.58, 0.65))
+	column.add_child(hint)
+
+func _add_button(parent: Control, text: String, node_name: String, cb: Callable) -> Button:
+	var button := Button.new()
+	button.name = node_name
+	button.text = text
+	button.custom_minimum_size = Vector2(320, 44)
+	button.pressed.connect(cb)
+	parent.add_child(button)
+	return button
+
+func open() -> void:
+	visible = true
+	_root.visible = true
+	Input.set_mouse_mode(Input.MOUSE_MODE_VISIBLE)
+	if _resume_button:
+		_resume_button.grab_focus()
+
+func close() -> void:
+	visible = false
+	_root.visible = false
+
+func _unhandled_input(event: InputEvent) -> void:
+	if not visible:
+		return
+	if event is InputEventKey and event.pressed and not event.echo and event.keycode == KEY_ESCAPE:
+		resume_requested.emit()
+		get_viewport().set_input_as_handled()