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/gamedelta.ts

@@ -1,4 +1,4 @@
-// Pure reducers that advance the per-game cache from live events (R4), so the UI renders a move
+// Pure reducers that advance the per-game cache from live events, so the UI renders a move
 // from the event without a follow-up game.state + game.history fetch. They never touch the network
 // or the cache store — the stream handler applies the returned cache and the game screen acts on
 // `refetch` — which keeps the gap / own-move / idempotency logic unit-testable in isolation.
@@ -42,7 +42,7 @@ export function applyMoveDelta(cached: CachedGame | undefined, d: MoveDelta): De
   // Nothing cached to advance (the game was never opened on this device): ignore it; the next open
   // cold-loads the game.
   if (!cached) return { refetch: false };
-  // A pre-R4 peer, or a dropped payload, carried no delta: the open game must refetch.
+  // An older peer, or a dropped payload, carried no delta: the open game must refetch.
   if (!d.move || !d.game) return { refetch: true };
   const have = cached.view.game.moveCount;
   const next = d.game.moveCount;