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/Header.svelte

@@ -110,7 +110,7 @@
      (--tg-safe-top) and --tg-content-top. Our header spans that full band (so the layout below
      is unchanged) and centres the title + menu as a pair (hamburger right of the title) within
      it, BELOW the notch — lining them up vertically with Telegram's own back/menu controls,
-     which sit in the band's corners (Stage 17). */
+     which sit in the band's corners. */
   :global(html.tg-fullscreen) .bar {
     min-height: var(--tg-content-top);
     box-sizing: border-box;
@@ -119,7 +119,7 @@
     /* +12px of vertical breathing room (6 above / 6 below the centred content, on top of the
        notch) so Telegram's native controls aren't flush against our header. Applied as
        padding because the bar is sized by its content here, not by min-height (owner review
-       tweaks, Stage 17). */
+       tweaks). */
     padding-top: calc(var(--tg-safe-top) + 6px);
     padding-bottom: 6px;
   }

ui/src/components/Modal.svelte

@@ -24,7 +24,7 @@
   // bottomSheet anchors a tall sheet (the chat) to the bottom and lifts it above the
   // keyboard with a transform (kb), driven by the visual viewport — a compositor-only
   // move, so neither the page behind nor the sheet relayouts as the keyboard animates
-  // (Stage 17). The backdrop is not resized in this mode (no per-event reflow).
+  // The backdrop is not resized in this mode (no per-event reflow).
   let vh = $state(0);
   let top = $state(0);
   let kb = $state(0);
@@ -95,7 +95,7 @@
   }
   /* Bottom-sheet mode (the chat): a wide sheet pinned to the bottom that lifts above the
      soft keyboard via a transform (--kb) — compositor-only, so the page behind and the
-     sheet itself do not relayout as the keyboard animates (Stage 17). */
+     sheet itself do not relayout as the keyboard animates. */
   .backdrop.bottom {
     align-items: flex-end;
     padding: 0;

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 {