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.

ui/src/app.css

@@ -42,10 +42,10 @@
   --gap: 8px;
   --pad: 12px;
   /* Height Telegram's native nav overlays at the top in fullscreen; set from the SDK's
-     content-safe-area inset (Stage 17), 0 elsewhere. */
+     content-safe-area inset, 0 elsewhere. */
   --tg-content-top: 0px;
   /* Telegram device safe-area top (the notch); TG's own nav controls sit between it and
-     --tg-content-top, so the in-app header aligns to that band (Stage 17), 0 elsewhere. */
+     --tg-content-top, so the in-app header aligns to that band, 0 elsewhere. */
   --tg-safe-top: 0px;
   --font: system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial,
     "Noto Sans", "Liberation Sans", sans-serif;