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

deploy/caddy/Caddyfile

@@ -53,7 +53,7 @@
 	# The game SPA and the Connect edge are served by the gateway. Strip any
 	# client-supplied X-Scrabble-Honeypot here so the gateway only ever honours the
 	# tag the honeypot block sets below (a client cannot self-tag a real request).
-	@gateway path /app /app/* /telegram /telegram/* /vk /vk/* /dict/* /scrabble.edge.v1.Gateway/*
+	@gateway path /app /app/* /telegram /telegram/* /vk /vk/* /dict/* /metrics/* /scrabble.edge.v1.Gateway/*
 	handle @gateway {
 		reverse_proxy gateway:8081 {
 			header_up -X-Scrabble-Honeypot