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/Profile.svelte

@@ -49,7 +49,7 @@
   }
 
   // populate loads the editable form from the current profile. The profile screen is
-  // edited inline (no edit/cancel toggle, Stage 17), so this runs on mount and after a
+  // edited inline (no edit/cancel toggle), so this runs on mount and after a
   // link/merge swaps the active account.
   function populate() {
     const p = app.profile;
@@ -215,7 +215,7 @@
         </form>
       {/if}
 
-      <!-- Linking & merge (Stage 11). Shown to everyone, including guests, who
+      <!-- Linking & merge. Shown to everyone, including guests, who
            upgrade by binding their first identity. -->
       <section class="emailbox">
         <h3>{t('profile.linkAccount')}</h3>
@@ -246,7 +246,7 @@
         {/if}
       </section>
 
-      <!-- Logout is hidden for now (Stage 17) but kept wired — drop `hidden` to re-enable
+      <!-- Logout is hidden for now but kept wired — drop `hidden` to re-enable
            once its entry point is decided; logout() also still runs on an invalid session. -->
       <button class="logout" hidden onclick={() => logout()}>{t('login.title')} / logout</button>
     {/if}