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.

gateway/internal/transcode/encode.go

@@ -80,7 +80,7 @@ func encodeProfile(p backendclient.ProfileResp) []byte {
 	return b.FinishedBytes()
 }
 
-// encodeLinkResult builds a LinkResult payload (Stage 11). A switched-session token
+// encodeLinkResult builds a LinkResult payload. A switched-session token
 // (a guest initiator whose durable counterpart won) is carried as a nested Session
 // for the client to adopt; it is omitted otherwise. supportedLangs is the variant
 // gating set for that switched session — the link flows run on the web, so it is the
@@ -138,7 +138,7 @@ func encodeMoveResult(r backendclient.MoveResultResp) []byte {
 }
 
 // encodeState builds a StateView payload. The rack is a vector of alphabet indices and the
-// alphabet display table is included only when the backend returned it (Stage 13: the
+// alphabet display table is included only when the backend returned it (the
 // client requests it on a per-variant cache miss).
 func encodeState(s backendclient.StateResp) []byte {
 	b := flatbuffers.NewBuilder(512)