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.

pkg/telemetry/telemetry.go

@@ -8,8 +8,7 @@
 // required locally or in CI), "stdout" (debugging) and "otlp" (gRPC export to a
 // collector). The OTLP endpoint and security are taken from the standard
 // OTEL_EXPORTER_OTLP_* environment variables read by the SDK, so no bespoke
-// configuration is introduced; the collector itself is stood up with the deploy
-// (PLAN.md Stage 14).
+// configuration is introduced; the collector itself is stood up with the deploy.
 package telemetry
 
 import (