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/accountmerge/merge.go

@@ -2,7 +2,7 @@
 // transaction: it sums statistics and the hint wallet, ORs the paid flag, repoints
 // the secondary's identities, transfers its games/chat/complaints/invitations,
 // de-duplicates friends and blocks, and leaves the secondary as an audit tombstone
-// (accounts.merged_into). It is the data core of Stage 11 account linking & merge
+// (accounts.merged_into). It is the data core of account linking & merge
 // (ARCHITECTURE.md §4); session revocation and any session switch are orchestrated
 // one layer up (the link service), since the in-memory session cache lives there.
 package accountmerge