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.

.gitea/workflows/ci.yaml

@@ -1,6 +1,6 @@
 name: CI
 
-# Single gated pipeline for the test contour (Stage 16/17). Gitea cannot express
+# Single gated pipeline for the test contour. Gitea cannot express
 # cross-workflow `needs`, so the full test suite and the auto test-deploy live in
 # one workflow.
 #
@@ -9,9 +9,9 @@ name: CI
 # `development` or `master` (the full test suite — the merge gate) and on a push
 # to `development` (after a merge). The deploy job runs only for `development`
 # (PR or merge), so a PR into `master` is test-only; the prod deploy is a manual
-# workflow (Stage 18).
+# workflow.
 #
-# Path-conditional jobs (Stage 17): `unit`/`integration`/`ui` run only when their
+# Path-conditional jobs: `unit`/`integration`/`ui` run only when their
 # code changed (the `changes` job decides). Because a skipped required check would
 # block a merge under branch protection, the always-running `gate` job aggregates
 # their results and is the ONLY required status check; it passes when every
@@ -263,7 +263,7 @@ jobs:
           TELEGRAM_GAME_CHANNEL_ID_EN: ${{ vars.TEST_TELEGRAM_GAME_CHANNEL_ID_EN }}
           TELEGRAM_GAME_CHANNEL_ID_RU: ${{ vars.TEST_TELEGRAM_GAME_CHANNEL_ID_RU }}
           # The test contour always uses Telegram's test environment — pinned here,
-          # not an operator variable. Stage 18's prod workflow leaves it false.
+          # not an operator variable. The prod workflow leaves it false.
           TELEGRAM_TEST_ENV: "true"
           VITE_TELEGRAM_BOT_ID: ${{ vars.TEST_VITE_TELEGRAM_BOT_ID }}
           VITE_TELEGRAM_LINK: ${{ vars.TEST_VITE_TELEGRAM_LINK }}
@@ -301,7 +301,7 @@ jobs:
       - name: Probe the landing and the gateway through caddy
         run: |
           set -u
-          # Two probes through the contour caddy (R3 split): "/" is the static
+          # Two probes through the contour caddy: "/" is the static
           # landing container, "/app/" is the gateway-served SPA shell.
           for i in $(seq 1 20); do
             if docker run --rm --network edge alpine:3.20 wget -q -T 5 -O /dev/null http://scrabble/ &&