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.

gateway/internal/connector/client.go

@@ -84,7 +84,7 @@ func (c *Client) ValidateInitData(ctx context.Context, initData string) (User, e
 
 // ValidateLoginWidget verifies Telegram Login Widget data and returns the user
 // identity, mapping a connector InvalidArgument to ErrInvalidLoginWidget. It backs
-// the link.telegram edge operation (Stage 11).
+// the link.telegram edge operation.
 func (c *Client) ValidateLoginWidget(ctx context.Context, data string) (User, error) {
 	resp, err := c.c.ValidateLoginWidget(ctx, &telegramv1.ValidateLoginWidgetRequest{Data: data})
 	if err != nil {