test(offline): mock e2e for a device-local vs_ai game (C8)

A Playwright spec drives the whole offline flow in the mock build: force the
installed-PWA display mode, enter offline via the Settings toggle (its readiness check
fetches the dawgs), assert the blue chrome + online-games-hidden + Stats-disabled, then
create and play a device-local English vs_ai game with a pinned bag seed (deterministic
rack NEWYMAO) -- play the opening WAY across the centre, watch the robot reply with a
real move, and reload to confirm the IndexedDB replay.

Enabling infra (all e2e-only; nothing enters the production build):
- mock/client.ts fetchDict serves the per-variant dawgs from the preview build's
  /e2edict/ (was: threw 'unsupported in mock').
- scripts/e2e-dict.mjs copies the real dawgs into dist-e2e from E2E_DICT_DIR (the ui CI
  job fetches the scrabble-dictionary release like the Go jobs; local default: the
  sibling scrabble-solver/dawg); playwright.config runs it between build and preview.
- localgame/id.ts setForcedSeed + gateway.ts window.__mock.setLocalSeed: a mock-only
  seam to pin a local game's bag seed (tree-shaken from prod).
- ci.yaml ui job: fetch the dawgs + pass E2E_DICT_DIR to the e2e step.
- docs/TESTING.md: the offline e2e + the mock-dawg wiring.

Verified: check 0 / unit 482 / e2e 198 (both engines) / app entry 113.8/114.

ui/src/lib/localgame/id.ts

@@ -17,7 +17,20 @@ export function newLocalGameId(): string {
   return `${LOCAL_ID_PREFIX}${Date.now().toString(36)}-${Math.random().toString(36).slice(2, 10)}`;
 }
 
-/** randomSeed returns a random 32-bit seed for a new local game's deterministic bag. */
+// A test seam for the mock e2e: pinning the seed makes a local game deal a known rack, so a spec
+// can play a precomputed opening word. Only the mock build's window.__mock wires setForcedSeed
+// (see lib/gateway.ts); a production build never calls it, so randomSeed stays truly random.
+let forcedSeed: bigint | null = null;
+
+/** setForcedSeed pins the value randomSeed returns (the mock-e2e seam); pass null to restore
+ *  randomness. */
+export function setForcedSeed(seed: bigint | null): void {
+  forcedSeed = seed;
+}
+
+/** randomSeed returns a random 32-bit seed for a new local game's deterministic bag (or the pinned
+ *  seed when a test has set one via setForcedSeed). */
 export function randomSeed(): bigint {
+  if (forcedSeed !== null) return forcedSeed;
   return BigInt(Math.floor(Math.random() * 0xffffffff));
 }