R6(a): de-stage code, docs, READMEs; split stage6_test

Mechanical, behaviour-preserving removal of Stage N / TODO-N / phase (RN)
references from comments, doc-comments, service READMEs, the current-state docs
(ARCHITECTURE, FUNCTIONAL+_ru, TESTING, UI_DESIGN), config-file comments, and the
.fbs/.proto schema comments. PLAN.md / PRERELEASE.md / CLAUDE.md keep the stage
history.

- Rename the only stage-named identifiers: registerStage8 -> registerSocialOps,
  registerStage11 -> registerLinkOps (gateway transcode).
- Split stage6_test.go: TestEmailLoginFlow -> email_test.go,
  TestGuestAutoMatchLeavesNoStats (+ provisionGuest) -> account_test.go.
- Regenerated proto bindings (push.pb.go, telegram_grpc.pb.go) from the de-staged
  .proto comments; FB Go/TS bindings unchanged (flatc strips schema comments).

go build/vet/gofmt clean across modules; integration typecheck and pnpm check green.

deploy/grafana/dashboards/edge-ux.json

@@ -37,8 +37,8 @@
     },
     {
       "type": "timeseries",
-      "title": "Rate limiting — request rate vs rejections (R3)",
-      "description": "Aggregate only (no per-user labels, the Stage 12/17 discipline): total edge request rate against the limiter rejection rate by class. Per-key detail lives in the admin console's Throttled view.",
+      "title": "Rate limiting — request rate vs rejections",
+      "description": "Aggregate only (no per-user labels): total edge request rate against the limiter rejection rate by class. Per-key detail lives in the admin console's Throttled view.",
       "gridPos": { "h": 8, "w": 24, "x": 0, "y": 16 },
       "fieldConfig": { "defaults": { "unit": "reqps" }, "overrides": [] },
       "datasource": { "type": "prometheus", "uid": "prometheus" },