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/mock/client.ts

@@ -98,7 +98,7 @@ export class MockGateway implements GatewayClient {
 
   constructor() {
     // Seed the per-variant alphabet cache the rack, blank chooser and scoring read, so the
-    // mock-driven UI is alphabet-agnostic without a backend (Stage 13).
+    // mock-driven UI is alphabet-agnostic without a backend.
     seedMockAlphabets();
   }
 
@@ -324,7 +324,7 @@ export class MockGateway implements GatewayClient {
   }
   async complaint(): Promise<void> {}
 
-  // Hide a finished game from the caller's list (Stage 17): drop it from the in-memory store so a
+  // Hide a finished game from the caller's list: drop it from the in-memory store so a
   // subsequent gamesList omits it, mirroring the backend's per-account, finished-only rule.
   async hideGame(gameId: string): Promise<void> {
     const g = this.game(gameId);
@@ -333,7 +333,7 @@ export class MockGateway implements GatewayClient {
     this.drafts.delete(gameId);
   }
 
-  // --- draft (Stage 17): an in-memory composition store, so the reload/off-turn flow is
+  // --- draft: an in-memory composition store, so the reload/off-turn flow is
   // exercised without a backend. A committed move clears the actor's own draft, as on the server.
   async draftGet(gameId: string): Promise<string> {
     return this.drafts.get(gameId) ?? '';
@@ -470,7 +470,7 @@ export class MockGateway implements GatewayClient {
     Object.assign(this.profile, p);
     return { ...this.profile };
   }
-  // --- account linking & merge (Stage 11) ---
+  // --- account linking & merge ---
   async linkEmailRequest(_email: string): Promise<void> {}
   async linkEmailConfirm(email: string, _code: string): Promise<LinkResult> {
     // An address containing "merge" stands in for one already owned by another