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/codec.test.ts

@@ -21,7 +21,7 @@ import {
 } from './codec';
 
 describe('codec', () => {
-  it('round-trips a draft save request and view (Stage 17)', () => {
+  it('round-trips a draft save request and view', () => {
     const json = '{"rack_order":"1,0","board_tiles":[]}';
     const req = fb.DraftRequest.getRootAsDraftRequest(new ByteBuffer(encodeDraftSave('g1', json)));
     expect(req.gameId()).toBe('g1');
@@ -35,7 +35,7 @@ describe('codec', () => {
     expect(decodeDraftView(b.asUint8Array())).toBe('{"x":1}');
   });
 
-  it('encodes a SubmitPlayRequest with alphabet indices (Stage 13)', () => {
+  it('encodes a SubmitPlayRequest with alphabet indices', () => {
     setAlphabet('scrabble_en', [
       { index: 0, letter: 'a', value: 1 },
       { index: 1, letter: 'b', value: 3 },
@@ -268,10 +268,10 @@ describe('codec', () => {
   });
 });
 
-// Stage 13: the live play loop exchanges alphabet indices, mapped through the per-variant
+// The live play loop exchanges alphabet indices, mapped through the per-variant
 // table cached in lib/alphabet. Each test seeds the cache it needs (setAlphabet replaces
 // the whole table), so they are independent of order.
-describe('codec — alphabet on the wire (Stage 13)', () => {
+describe('codec — alphabet on the wire', () => {
   it('encodes an ExchangeRequest as alphabet indices, blank as the sentinel', () => {
     setAlphabet('scrabble_en', [
       { index: 0, letter: 'a', value: 1 },