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.

backend/internal/server/handlers_friends.go

@@ -9,7 +9,7 @@ import (
 	"github.com/google/uuid"
 )
 
-// The /api/v1/user/friends/* handlers wire the social friend graph (Stage 8): the
+// The /api/v1/user/friends/* handlers wire the social friend graph: the
 // befriend-an-opponent request flow, the one-time friend-code path, and the
 // friends/incoming lists. They follow handlers_user.go: X-User-ID identity, a domain
 // call, a JSON DTO. Account ids are projected to {id, display_name} refs resolved