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.

backend/internal/engine/engine.go

@@ -49,7 +49,7 @@ func (v Variant) String() string {
 
 // Language returns the variant's interface/bot language tag: "en" for English Scrabble, "ru" for
 // the Russian variants (Russian Scrabble and Erudite). It routes a game's out-of-app push to the
-// matching per-language Telegram bot — by the game, not the recipient's last-login bot (Stage 17).
+// matching per-language Telegram bot — by the game, not the recipient's last-login bot.
 func (v Variant) Language() string {
 	if v == VariantEnglish {
 		return "en"