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/lib/premiums.test.ts

@@ -4,7 +4,7 @@ import { BOARD_SIZE, centre, premiumGrid } from './premiums';
 // Premium-square geometry parity with scrabble-solver/rules/rules.go: scrabble_en/scrabble_ru
 // share standardBoard (centre is a double word); erudit_ru shares the geometry but a
 // non-doubling centre. Tile-value and alphabet parity moved to the Go engine test
-// (backend/internal/engine AlphabetTable) in Stage 13 — the server now owns that table.
+// (backend/internal/engine AlphabetTable) — the server now owns that table.
 describe('premium layout', () => {
   it('is a 15x15 grid with TW corners', () => {
     const g = premiumGrid('scrabble_en');