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/dict/loader.ts

@@ -8,6 +8,7 @@
 import { Dawg } from './dawg';
 import { dictKey, idbGetDawg, idbPutDawg, idbDelDawg, requestPersist } from './store';
 import { gateway } from '../gateway';
+import { noteDictFetched, noteDictCacheHit, noteDictMiss as noteDictMissMetric } from '../localeval-metrics';
 import type { Variant } from '../model';
 
 // Loaded readers by key. A dictionary is immutable, so an instance is reused for
@@ -31,6 +32,7 @@ let dictDisabled = false;
  */
 export function noteDictMiss(): void {
   if (!dictDisabled && ++misses >= MISS_LIMIT) dictDisabled = true;
+  noteDictMissMetric();
 }
 
 /** dictLoadingDisabled reports whether the bad-connection breaker has tripped for this
@@ -73,6 +75,7 @@ async function load(variant: Variant, version: string, key: string, signal?: Abo
     if (cached) {
       const reader = new Dawg(new Uint8Array(cached));
       instances.set(key, reader);
+      noteDictCacheHit();
       return reader;
     }
   } catch {
@@ -101,6 +104,7 @@ async function load(variant: Variant, version: string, key: string, signal?: Abo
     return null; // not a dawg — do not cache it
   }
   instances.set(key, reader);
+  noteDictFetched();
   void idbPutDawg(key, buf);
   void requestPersist();
   return reader;