feat(gateway): unsupported-engine telemetry beacon + Grafana counter

The index.html boot guard now fires a fire-and-forget beacon (POST /telemetry/unsupported)
when it turns a client away on the unsupported-engine screen, so the owner can see — on the
Users dashboard beside "app opens" — how many real clients hit it and on which engines.

- Client: navigator.sendBeacon (fetch fallback) with a localStorage dedup keyed by app version
  + reason + Chromium, so a user reopening the app is one report, not ten.
- Gateway: a new unauthenticated POST /telemetry/unsupported handler (the client never booted,
  so it carries no session), per-IP public-limited and body-capped, mirroring the export-download
  route. It folds the beacon into the OTel counter unsupported_engine_total {reason =
  no_bigint/no_proxy/boot_error/other, chromium}, with reason allow-listed and the Chromium major
  reduced to a bounded range (normalizeUnsupported) so a spoofed beacon cannot inflate the metric
  cardinality; the full user agent is logged, not labelled.
- Caddy: /telemetry/* added to the @gateway matcher (else it falls to the landing catch-all).
- Grafana: two panels on the Scrabble — Users dashboard (by reason, by Chromium major).
- Docs: ARCHITECTURE.md §11.

Tests: recordUnsupportedEngine (metric split), normalizeUnsupported (cardinality bounding), and
the handler route end to end (204 / 405 / counter). go build+vet+test and gofmt clean; ui
check/build/e2e green; the client beacon + dedup verified against a forced hard-gate (BigInt
removed) — one POST, deduped on reopen, correct reason/chromium/version labels.

deploy/caddy/Caddyfile

@@ -86,7 +86,7 @@
 	# 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/* /dl/* /metrics/* /scrabble.edge.v1.Gateway/*
+	@gateway path /app /app/* /telegram /telegram/* /vk /vk/* /dict/* /dl/* /metrics/* /telemetry/* /scrabble.edge.v1.Gateway/*
 	handle @gateway {
 		reverse_proxy gateway:8081 {
 			header_up -X-Scrabble-Honeypot

deploy/grafana/dashboards/users.json

@@ -56,6 +56,22 @@
       "gridPos": { "h": 8, "w": 24, "x": 0, "y": 24 },
       "datasource": { "type": "prometheus", "uid": "prometheus" },
       "targets": [{ "refId": "A", "expr": "sum(rate(local_eval_preview_total[1h])) by (path)", "legendFormat": "{{path}}" }]
+    },
+    {
+      "type": "timeseries",
+      "title": "Unsupported-engine screens (cumulative by reason)",
+      "description": "Clients turned away by the index.html boot guard because the engine cannot run the app (an old Android System WebView), by reason: no_bigint / no_proxy (a missing unpolyfillable primitive) or boot_error (an uncaught startup failure). Deduped per device/version, so this counts distinct blocked installs, not launches. The effective floor is Chrome 67 (BigInt).",
+      "gridPos": { "h": 8, "w": 12, "x": 0, "y": 32 },
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "targets": [{ "refId": "A", "expr": "sum(unsupported_engine_total) by (reason)", "legendFormat": "{{reason}}" }]
+    },
+    {
+      "type": "timeseries",
+      "title": "Unsupported engines by Chromium (rate)",
+      "description": "The same blocked clients by reported Chromium major version — which old in-app WebViews are still in the wild. \"other\" is an unparseable or out-of-range version.",
+      "gridPos": { "h": 8, "w": 12, "x": 12, "y": 32 },
+      "datasource": { "type": "prometheus", "uid": "prometheus" },
+      "targets": [{ "refId": "A", "expr": "sum(rate(unsupported_engine_total[1h])) by (chromium)", "legendFormat": "Chromium {{chromium}}" }]
     }
   ]
 }