feat(offline): randomise the hotseat seating (random first mover)

The 2-4 hotseat players now start in a random seating order (so who goes
first, and the turn order, is random) — matching the online games. The
engine starts at seat 0, so shuffling the seats at creation picks the
starter; the shuffle is seed-driven (a distinct derivation from the tile
bag), so replay reproduces it. Scoped to hotseat — vs_ai stays human-first.
- lib/roster.ts: shuffleSeeded (unit-tested); NewGame shuffles buildSeats
  with the game seed before create.
- e2e: pin a seed that keeps the roster order so the lock assertions stay
  deterministic.

ui/e2e/hotseat.spec.ts

@@ -41,8 +41,10 @@ test.describe('offline hotseat (pass-and-play)', () => {
     await enterLobby(page);
     await goOffline(page);
 
-    // A known bag seed so the locked seat deals a full rack deterministically.
-    await page.evaluate(() => (window as unknown as { __mock: { setLocalSeed(s: string): void } }).__mock.setLocalSeed('1'));
+    // A known seed: the bag deals a full rack deterministically, AND the seeded seating shuffle keeps
+    // the roster order (Ann first) so the locked-seat assertions below are stable. (Seat order is
+    // randomised at start; the shuffle is seed-driven — seed '1' would seat Bob first, '2' Ann first.)
+    await page.evaluate(() => (window as unknown as { __mock: { setLocalSeed(s: string): void } }).__mock.setLocalSeed('2'));
 
     // New game -> the offline mode selector now offers "with friends" (hotseat).
     await page.locator('button.tab').nth(0).click();