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/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;