PauseOverlay class

Pause panel wired directly to GameSessionController — the pause button/system-lifecycle-pause round trip this repo already has (see game_session_controller.dart's own RoyLifecycleCoordinator hook) gets exactly one visible UI on top of it. Caller mounts this unconditionally inside their own Stack (same "always mounted, NeonDialog.overlaySlot animates the panel in/out" convention as every other in-tree overlay in this kit) — it decides on its own, from session, whether to show:

• Visible when GameSessionController.snapshot is GameSessionPhase.paused AND GamePauseReason.user is one of the active reasons — a background-triggered GamePauseReason.system pause does NOT show this by default (a player didn't ask to see a pause menu just because the app backgrounded), unless showForSystemPause.
• The Android/back gesture resumes instead of popping the underlying route while visible (a real PopScope, not the show()-then-pop trick NeonDialog.show uses for a route dialog — this overlay is never a route, exactly like NeonDialog.overlay/.overlaySlot).
• onResume/onRestart/onSettings/onQuit are overridable; leaving onResume/onRestart unset falls back to GameSessionController.resume/GameSessionController.restart. onSettings/onQuit have no sensible package-level default (the kit doesn't know a game's settings screen or what "quit" means for it) — leaving either null hides that one button instead of wiring a no-op.

Focus: the panel is wrapped in FocusTrapScope (FEAT-82), which autofocuses into the panel as soon as it becomes visible AND restores focus to whatever had it right before, the moment the panel is dismissed — a keyboard/TV-remote/gamepad/screen-reader user lands on the pause menu and gets their exact place back afterward, not wherever FocusManager happens to fall back to. Every button inside (via CommonButton's own PressableScale) is real-keyboard- activatable (Enter/Space/gamepad A), not gesture-only — closes the gap this doc comment used to flag as out of scope for FEAT-53.

Game-time coordination is automatic and needs no wiring here: a GameTimeController built with session: this same session already freezes itself from GameSessionController's own pause state (FEAT-49). Audio is deliberately NOT auto-paused/resumed by this widget — whether background music should keep playing behind a pause menu is a per-game call this kit shouldn't make for every consumer; pause/resume AudioManager yourself in onResume/wherever you call GameSessionController.pause if your game wants that.