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.

deploy/docker-compose.yml

@@ -17,7 +17,7 @@
 #   - `edge` (external): the host caddy reaches this contour at `scrabble:80`
 #     (the in-compose caddy's alias). The in-compose caddy terminates only HTTP in
 #     the test contour; the host caddy terminates TLS and forwards. For prod
-#     (Stage 18, no host caddy) set CADDY_SITE_ADDRESS to the domain so the caddy
+#     (no host caddy) set CADDY_SITE_ADDRESS to the domain so the caddy
 #     does its own ACME — the contour is then self-contained.
 #   - The connector egresses to api.telegram.org through the `vpn` sidecar
 #     (network_mode: service:vpn); it answers internal gRPC at `telegram:9091`.
@@ -102,7 +102,7 @@ services:
     networks: [internal]
 
   # --- Landing (static) -------------------------------------------------------
-  # The public landing page in its own caddy container (R3): the contour caddy
+  # The public landing page in its own caddy container: the contour caddy
   # routes the catch-all (notably /) here, the gateway keeps only /app/,
   # /telegram/ and the Connect edge. Shares the gateway Dockerfile's UI build
   # stage — identical build args keep that stage a single cached build.