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/ChatScreen.svelte

@@ -7,7 +7,7 @@
   import { t } from '../lib/i18n/index.svelte';
   import type { ChatMessage, StateView } from '../lib/model';
 
-  // The chat is its own screen (Stage 17), so the soft keyboard simply resizes the viewport with
+  // The chat is its own screen, so the soft keyboard simply resizes the viewport with
   // the input pinned to the bottom — no modal relayout jank. It loads the game state (for the
   // turn-based chat/nudge toggle) and the message list, and clears the unread badge while open.
   let { id }: { id: string } = $props();