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/webui/webui.go

@@ -3,13 +3,13 @@
 // The committed dist/ holds only a placeholder index.html so the gateway module
 // compiles with a plain `go build` (and in CI) without a UI build. The production
 // gateway image replaces dist/ with the real Vite build — minus landing.html, which
-// ships in the separate landing container since R3 — before compiling (see
+// ships in the separate landing container — before compiling (see
 // gateway/Dockerfile), so the binary ships the UI inside it. Because Vite is built
 // with a relative asset base, one build serves under any path: the game SPA is
 // mounted at /app/ (web) and /telegram/ (the Telegram Mini App) — the single-origin
 // model in docs/ARCHITECTURE.md §13.
 //
-// Caching (Stage 17): Vite emits hash-named files under assets/, so those are immutable and
+// Caching: Vite emits hash-named files under assets/, so those are immutable and
 // cached hard (a reload/relaunch is a cache hit, not a re-download); the HTML shells carry
 // no-cache so a new deploy is picked up immediately.
 package webui