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/game/Game.svelte

@@ -8,6 +8,7 @@
   import Board from './Board.svelte';
   import Rack from './Rack.svelte';
   import { gateway } from '../lib/gateway';
+  import { notePreviewLocal, notePreviewNetwork } from '../lib/localeval-metrics';
   import { navigate } from '../lib/router.svelte';
   import { app, handleError, showToast, markChatRead, seedChatUnread } from '../lib/app.svelte';
   import { connection } from '../lib/connection.svelte';
@@ -725,6 +726,7 @@
       if (reader && hasAlphabet(v.game.variant)) {
         try {
           preview = d.evaluateLocal(reader, v.game.variant, board, sub.tiles, v.game.multipleWordsPerTurn);
+          notePreviewLocal();
           return;
         } catch {
           /* fall through to the network preview */
@@ -736,6 +738,7 @@
     previewTimer = setTimeout(async () => {
       try {
         preview = await gateway.evaluate(id, sub.tiles, variant, ctrl.signal);
+        notePreviewNetwork();
       } catch {
         /* best-effort (or aborted) */
       }