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/transport.ts

@@ -80,6 +80,18 @@ export function createTransport(baseUrl: string): GatewayClient {
       return res.arrayBuffer();
     },
 
+    async reportLocalEval(counts) {
+      if (!token) throw new Error('reportLocalEval: no session');
+      // keepalive so a batch flushed as the app is backgrounded still reaches the edge.
+      const res = await fetch(`${origin}/metrics/local-eval`, {
+        method: 'POST',
+        headers: { authorization: `Bearer ${token}`, 'content-type': 'application/json' },
+        body: JSON.stringify(counts),
+        keepalive: true,
+      });
+      if (!res.ok) throw new Error(`reportLocalEval: HTTP ${res.status}`);
+    },
+
     async authTelegram(initData) {
       return codec.decodeSession(await exec('auth.telegram', codec.encodeTelegramLogin(initData, browserOffset())));
     },