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/scripts/bundle-size.mjs

@@ -10,10 +10,9 @@
 //   - shared (svelte+i18n):  near-static framework runtime; only drifts on a dep/Svelte bump.
 //   - landing own:           the landing's own code; kept minimal.
 // Today ~74 KB (app entry) + ~23 KB (shared) = ~97 KB for the app; the landing's own chunk is
-// ~2 KB. Lazy-loading was analysed and rejected for R5 (no total-size win — every chunk still
+// ~2 KB. Lazy-loading was analysed and rejected (no total-size win — every chunk still
 // ships and is summed — plus added request latency); the bulk is the Connect/FlatBuffers
-// transport runtime + generated bindings + the Svelte runtime, irreducible within scope. See
-// PRERELEASE.md R5 for the full rationale.
+// transport runtime + generated bindings + the Svelte runtime, irreducible within scope.
 import { readFileSync, existsSync } from 'node:fs';
 import { gzipSync } from 'node:zlib';
 import { join } from 'node:path';