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/transcode_draft_test.go

@@ -13,7 +13,7 @@ import (
 )
 
 // TestDraftSaveForwardsRawJSON checks the save handler forwards the client's composition JSON
-// to the backend verbatim (the "no double-encode" contract, Stage 17) with the user header.
+// to the backend verbatim (the "no double-encode" contract) with the user header.
 func TestDraftSaveForwardsRawJSON(t *testing.T) {
 	const body = `{"rack_order":"1,0","board_tiles":[{"row":7,"col":7,"letter":"Q","blank":false}]}`
 	backend, cleanup := fakeBackend(t, func(w http.ResponseWriter, r *http.Request) {