feat(hint): persist the vs_ai idle-hint wait (wall-clock + read sanitiser)

Per the owner's call, the idle-hint gate now PERSISTS across leaving and reopening the
app, instead of the session-scoped monotonic clock: the unlock is a wall-clock instant
(hintUnlockAtMs) stamped from the robot's reply, stored on the local record, carried on
the game view + the opponent_moved move delta, and read through a sanitiser that caps it
at now + the window. So:
- the wait survives a relaunch (a stuck turn is not forgotten);
- a device clock set BACK cannot freeze the gate (the cap bounds the remaining to the
  window and self-heals on the next read);
- a clock set FORWARD just opens the hint early -- accepted as harmless for a solo game.

- lib/hints.ts hintGateRemainingMs now takes the unlock instant + clamps to the window.
- localgame: re-add hintUnlockAtMs to the record/meta; stamp it off the robot's reply;
  sanitise on read (a shared hintUnlock helper feeds stateView + the opponent_moved event).
- gamedelta + PushEvent: the move delta carries hintUnlockAtMs so the view stays fresh.
- Game.svelte: hintUnlockAt derives from the view; the tick + lock + toast unchanged.
- offline.spec.ts: also assert the lock survives a reload (the wait persisted).
- Docs FUNCTIONAL(+_ru)/ARCHITECTURE updated (persist + sanitiser, not monotonic-resets).

ui/src/lib/gamedelta.ts

@@ -11,6 +11,9 @@ export interface MoveDelta {
   move?: MoveRecord;
   game?: GameView;
   bagLen: number;
+  /** For a vs_ai game, the refreshed idle-hint unlock instant (the robot's reply re-arms the gate),
+   *  so the cached view stays current without a refetch; absent leaves the prior value. */
+  hintUnlockAtMs?: number;
 }
 
 /**
@@ -50,7 +53,12 @@ export function applyMoveDelta(cached: CachedGame | undefined, d: MoveDelta): De
   if (next > have + 1) return { refetch: true }; // a gap — an intermediate move was missed
   // The actor's own move changed their rack (a draw), which opponent_moved does not carry.
   if (d.move.player === cached.view.seat) return { refetch: true };
-  const view: StateView = { ...cached.view, game: d.game, bagLen: d.bagLen };
+  const view: StateView = {
+    ...cached.view,
+    game: d.game,
+    bagLen: d.bagLen,
+    hintUnlockAtMs: d.hintUnlockAtMs ?? cached.view.hintUnlockAtMs,
+  };
   return { cache: { view, moves: [...cached.moves, d.move] }, refetch: false };
 }
 
@@ -79,7 +87,7 @@ export function applyGameOver(cached: CachedGame | undefined, game: GameView | u
  */
 export function advanceCached(cached: CachedGame | undefined, e: PushEvent): CachedGame | undefined {
   if (e.kind === 'opponent_moved') {
-    return applyMoveDelta(cached, { move: e.move, game: e.game, bagLen: e.bagLen }).cache;
+    return applyMoveDelta(cached, { move: e.move, game: e.game, bagLen: e.bagLen, hintUnlockAtMs: e.hintUnlockAtMs }).cache;
   }
   if (e.kind === 'game_over') return applyGameOver(cached, e.game).cache;
   if (e.kind === 'opponent_joined' && e.state) return applyOpponentJoined(cached, e.state);