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/inttest/draft_test.go

@@ -34,7 +34,7 @@ func newDraftGame(t *testing.T) (*game.Service, uuid.UUID, []uuid.UUID, engine.M
 	return svc, g.ID, seats, hint
 }
 
-// TestDraftPersistAndConflictReset covers Stage 17 draft persistence: a round-trip of the
+// TestDraftPersistAndConflictReset covers draft persistence: a round-trip of the
 // rack order + board tiles, the actor's own draft cleared on their move, and an opponent's
 // board draft reset when a committed play overlaps one of its cells (the rack order kept).
 func TestDraftPersistAndConflictReset(t *testing.T) {