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.

ui/src/lib/result.test.ts

@@ -51,7 +51,7 @@ describe('resultBadge', () => {
 
   it('finished two-player: a 0-0 resignation is a defeat, not a score-tied win', () => {
     // The opponent won by resignation (isWinner) although neither side scored — the lobby
-    // must read this as a loss, matching the game-detail screen (Stage 17 regression).
+    // must read this as a loss, matching the game-detail screen (regression).
     expect(resultBadge(game([seat(0, 'me', 0), seat(1, 'a', 0, true)]), 'me')).toEqual({
       key: 'result.defeat',
       emoji: '🥈',