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.

ui/src/lib/mock/data.ts

@@ -42,7 +42,7 @@ export const PROFILE: Profile = {
 };
 
 // Seed social/account data for the mock (pnpm start + Playwright). The mock profile
-// is a durable account so the Stage 8 surfaces (friends, stats, history) are reachable.
+// is a durable account so the social surfaces (friends, stats, history) are reachable.
 // Ann is the active game's opponent but deliberately not a friend, so the in-game
 // "add to friends" flow is demonstrable; Kaya (a finished-game opponent) is the friend.
 export const MOCK_FRIENDS: AccountRef[] = [{ accountId: 'kaya', displayName: 'Kaya' }];