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/screens/NewGame.svelte

@@ -12,8 +12,8 @@
   // The auto-match move clock (mirrors backend game.DefaultTurnTimeout = 24h).
   const AUTO_MATCH_HOURS = 24;
 
-  // The offered variants are gated by the languages the sign-in service supports
-  // (Stage 15); the auto-match list and the friend-invite picker both use this.
+  // The offered variants are gated by the languages the sign-in service supports;
+  // the auto-match list and the friend-invite picker both use this.
   const variants = $derived(availableVariants(app.session?.supportedLanguages));
   const timeouts = [
     { secs: 300, key: 'time.minutes' as MessageKey, n: 5 },
@@ -39,7 +39,7 @@
     }
   }
   // startPoll is the matchmaking fallback used only while the live stream is down: with the stream
-  // up the match_found push drives navigation (R4). It polls lobby.poll every 2.5s.
+  // up the match_found push drives navigation. It polls lobby.poll every 2.5s.
   function startPoll() {
     if (poll) return;
     poll = setInterval(async () => {
@@ -58,7 +58,7 @@
   }
   // cancelSearch leaves the auto-match pool on the backend too, so a cancelled quick-match
   // is actually dequeued — otherwise the account stays queued (blocking a re-queue) and the
-  // reaper later substitutes a robot for a game the player abandoned (Stage 17 fix).
+  // reaper later substitutes a robot for a game the player abandoned.
   function cancelSearch() {
     stop();
     searching = false;
@@ -104,7 +104,7 @@
   let selected = $state<string[]>([]);
   let friendFilter = $state('');
   // No default game type yet — the player must pick one (a smarter default from play
-  // history / language is TODO-6). '' renders the disabled placeholder option.
+  // history / language would be a future refinement). '' renders the disabled placeholder option.
   let inviteVariant = $state<Variant | ''>('');
   let timeoutSecs = $state(86400);
   let hints = $state(1);