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;
   }