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/game/CheckScreen.svelte

@@ -8,7 +8,7 @@
   import { canCheckWord, sanitizeCheckWord } from '../lib/checkword';
   import type { Variant } from '../lib/model';
 
-  // Word-check on its own screen (Stage 17): unlimited dictionary lookups, each with a
+  // Word-check on its own screen: unlimited dictionary lookups, each with a
   // complaint, off the board so the soft keyboard never relayouts the play area.
   let { id }: { id: string } = $props();