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/.env.example

@@ -1,6 +1,6 @@
 # Environment for deploy/docker-compose.yml. The CI deploy job (ci.yaml) maps the
-# Gitea TEST_-prefixed secrets/variables onto these unprefixed names; Stage 18
-# maps the PROD_-prefixed set the same way. Copy to deploy/.env for a local run.
+# Gitea TEST_-prefixed secrets/variables onto these unprefixed names; the prod
+# deploy maps the PROD_-prefixed set the same way. Copy to deploy/.env for a local run.
 #
 # Full reference (required vs optional, defaults, secret-vs-variable): deploy/README.md.
 
@@ -17,7 +17,7 @@ LOG_LEVEL=info
 
 # --- Edge / caddy -----------------------------------------------------------
 # Test: ":80" (the host caddy terminates TLS and forwards to scrabble:80 on the
-# external `edge` network). Prod (Stage 18): a domain so caddy does its own ACME.
+# external `edge` network). Prod: a domain so caddy does its own ACME.
 CADDY_SITE_ADDRESS=:80
 GM_BASICAUTH_USER=gm
 GM_BASICAUTH_HASH=                     # required; `caddy hash-password` bcrypt hash