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.

pkg/README.md

@@ -16,12 +16,12 @@ fbs/scrabblefb/   # committed generated Go for the schema
 - **`proto/push/v1`** is the single gRPC server-stream the backend exposes and
   the gateway subscribes to (`Event{user_id, kind, payload, event_id}`); the
   `payload` is an opaque FlatBuffers body the gateway forwards verbatim.
-- **`proto/telegram/v1`** is the Telegram connector's RPC contract (Stage 9; Stage 11
-  added `ValidateLoginWidget` for the web Login Widget sign-in).
+- **`proto/telegram/v1`** is the Telegram connector's RPC contract (including
+  `ValidateLoginWidget` for the web Login Widget sign-in).
 - **`fbs`** holds the client↔gateway request/response and event payloads as
   FlatBuffers tables. The backend encodes the push payloads from these types; the
   gateway transcodes the rest to and from the backend's JSON; the UI generates
-  TypeScript from the same `.fbs` (Stage 7).
+  TypeScript from the same `.fbs`.
 
 ## Generated code