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.

platform/telegram/internal/render/render.go

@@ -53,7 +53,7 @@ func Render(kind string, payload []byte, lang string) (Message, bool) {
 	return Message{}, false
 }
 
-// yourTurnText renders the enriched "your turn" body (Stage 17), voiced as the opponent who
+// yourTurnText renders the enriched "your turn" body, voiced as the opponent who
 // just moved ("{name}: my move — «WORD». Score 120:95"). It falls back to the plain phrase when
 // the opponent name is missing (an older backend, or an unresolved name).
 func yourTurnText(ev *scrabblefb.YourTurnEvent, p phrases) string {
@@ -76,7 +76,7 @@ func yourTurnText(ev *scrabblefb.YourTurnEvent, p phrases) string {
 	}
 }
 
-// gameOverText renders the "game over" body (Stage 17) from the recipient's own perspective.
+// gameOverText renders the "game over" body from the recipient's own perspective.
 func gameOverText(ev *scrabblefb.GameOverEvent, p phrases) string {
 	score := string(ev.ScoreLine())
 	switch string(ev.Result()) {