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/components/Screen.svelte

@@ -31,11 +31,11 @@
 
   // The promotional banner is feature-gated OFF until it is polished after release. The flag is
   // a compile-time `false`, so the {#if} branch — and with it the AdBanner import and its
-  // banner.ts logic — is dead-code-eliminated from the production bundle (Stage 17). Flip to
+  // banner.ts logic — is dead-code-eliminated from the production bundle. Flip to
   // true to bring it back.
   const SHOW_AD_BANNER = false;
 
-  // Edge-swipe back (Stage 17): a left-edge rightward drag returns to `back`, the standard
+  // Edge-swipe back: a left-edge rightward drag returns to `back`, the standard
   // mobile gesture. Listened at the window in the CAPTURE phase so the board's own pointer
   // handlers (which capture/stop the event) can never swallow it; armed only from the very
   // left edge (<=24px), touch/pen only, so it never competes with the board's gestures.
@@ -72,7 +72,7 @@
     flex-direction: column;
     /* Fit the visible viewport (set from visualViewport, app.svelte.ts) so a screen with a
        bottom input — chat, word-check — stays above an open soft keyboard without the page
-       scrolling; falls back to the full height where the var is unset (Stage 17). */
+       scrolling; falls back to the full height where the var is unset. */
     height: var(--vvh, 100%);
   }
   .content {