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/Dockerfile

@@ -42,7 +42,7 @@ COPY ui ./
 RUN pnpm build
 
 # --- landing -------------------------------------------------------------------
-# The public landing page as its own static container (R3): the same Vite build
+# The public landing page as its own static container: the same Vite build
 # served by caddy at /, so stray public traffic is absorbed by static file
 # serving and never reaches the Go edge.
 FROM caddy:2-alpine AS landing
@@ -58,7 +58,7 @@ COPY gateway ./gateway
 
 # Replace the committed placeholder with the freshly built UI before compiling, so
 # go:embed bakes the real bundle into the binary. The landing shell ships in the
-# landing image, not in the gateway (R3).
+# landing image, not in the gateway.
 RUN rm -rf gateway/internal/webui/dist
 COPY --from=ui /ui/dist gateway/internal/webui/dist
 RUN rm gateway/internal/webui/dist/landing.html