feat(telemetry): local move-preview adoption metrics (Phase 4)

Measure uptake of the client-side local move-preview accelerator (§5) so
adoption can be watched before defaulting it on: app cold starts, dictionary
loads by result (fetched / cache_hit / miss) and move previews by path
(local / network — the backend load shed).

A small best-effort client beacon (POST /metrics/local-eval, session-gated)
batches counter deltas and posts them on a 60s timer and when the app is
backgrounded — never on the gameplay path: the in-app counters are plain
in-memory increments, only the periodic flush touches the network and it is
fire-and-forget. The gateway folds each batch into three OTel counters
(local_eval_cold_start_total, local_eval_dict_load_total,
local_eval_preview_total), clamped against a spoofed inflation.

- gateway: counters + recordLocalEval + session-gated /metrics/local-eval handler
- ui: localeval-metrics accumulator/beacon; hooks in the dict loader, in
  Game.recompute and in bootstrap (skipped under the mock harness)
- caddy: route /metrics/* to the gateway
- docs: ARCHITECTURE §11; Grafana "Scrabble — Users" dashboard panels

ui/src/lib/localeval-metrics.ts

@@ -0,0 +1,84 @@
+// Client-side accumulator + beacon for local move-preview adoption telemetry. It counts a
+// few events — the app cold start, dictionary loads by result, and move previews by path —
+// and posts the running deltas to the edge periodically and when the app is backgrounded,
+// so the backend can chart adoption (docs/ARCHITECTURE.md §11). Best-effort throughout: a
+// failed beacon keeps its counts to retry, and nothing here throws into the caller. It lives
+// outside the lazy dict subsystem so the cold start is recorded even when no game is opened.
+
+import { gateway } from './gateway';
+
+interface Counts {
+  cold_start: number;
+  dict_fetched: number;
+  dict_cache_hit: number;
+  dict_miss: number;
+  preview_local: number;
+  preview_network: number;
+}
+
+function zero(): Counts {
+  return { cold_start: 0, dict_fetched: 0, dict_cache_hit: 0, dict_miss: 0, preview_local: 0, preview_network: 0 };
+}
+
+let counts = zero();
+let started = false;
+
+function total(c: Counts): number {
+  return c.cold_start + c.dict_fetched + c.dict_cache_hit + c.dict_miss + c.preview_local + c.preview_network;
+}
+
+// flush posts the accumulated deltas and resets them; on failure it folds the batch back in
+// so the next flush retries. Best-effort.
+async function flush(): Promise<void> {
+  if (total(counts) === 0) return;
+  const batch = counts;
+  counts = zero();
+  try {
+    await gateway.reportLocalEval({ ...batch });
+  } catch {
+    counts.cold_start += batch.cold_start;
+    counts.dict_fetched += batch.dict_fetched;
+    counts.dict_cache_hit += batch.dict_cache_hit;
+    counts.dict_miss += batch.dict_miss;
+    counts.preview_local += batch.preview_local;
+    counts.preview_network += batch.preview_network;
+  }
+}
+
+/**
+ * startLocalEvalMetrics records the app cold start and begins the periodic beacon. Called once
+ * at launch (a repeat call is a no-op). It also flushes when the app is backgrounded, so a
+ * batch is not lost when the tab is closed.
+ */
+export function startLocalEvalMetrics(): void {
+  if (started) return;
+  started = true;
+  counts.cold_start += 1;
+  setInterval(() => void flush(), 60_000);
+  if (typeof document !== 'undefined') {
+    document.addEventListener('visibilitychange', () => {
+      if (document.visibilityState === 'hidden') void flush();
+    });
+  }
+}
+
+/** noteDictFetched: a dictionary was downloaded and cached (an IndexedDB fill). */
+export function noteDictFetched(): void {
+  counts.dict_fetched += 1;
+}
+/** noteDictCacheHit: a dictionary loaded from the IndexedDB cache (no download). */
+export function noteDictCacheHit(): void {
+  counts.dict_cache_hit += 1;
+}
+/** noteDictMiss: a warm-up did not produce a usable dictionary (failed or timed out). */
+export function noteDictMiss(): void {
+  counts.dict_miss += 1;
+}
+/** notePreviewLocal: a move preview was computed on-device. */
+export function notePreviewLocal(): void {
+  counts.preview_local += 1;
+}
+/** notePreviewNetwork: a move preview fell back to the network. */
+export function notePreviewNetwork(): void {
+  counts.preview_network += 1;
+}