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.

platform/telegram/internal/loginwidget/validator.go

@@ -1,6 +1,6 @@
 // Package loginwidget validates Telegram Login Widget authorization data, the
 // web (non-Mini-App) sign-in flow used to attach a Telegram identity to an existing
-// account during linking (Stage 11). Like initdata it lives in the connector
+// account during linking. Like initdata it lives in the connector
 // because the secret is derived from the bot token, held only here
 // (ARCHITECTURE.md §12); the gateway calls the connector's ValidateLoginWidget RPC.
 //