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.

backend/internal/game/draft.go

@@ -21,7 +21,7 @@ type DraftTile struct {
 	Blank  bool   `json:"blank"`
 }
 
-// Draft is a player's persisted client-side composition for a game (Stage 17): the
+// Draft is a player's persisted client-side composition for a game: the
 // preferred rack tile order and the board tiles laid but not yet submitted. The server
 // keeps it so a reload or a second device resumes the same arrangement.
 type Draft struct {
@@ -101,7 +101,7 @@ func (s *Store) clearDraft(ctx context.Context, gameID, accountID uuid.UUID) err
 
 // resetConflictingBoardDrafts clears the board_tiles of every OTHER player's draft that has
 // a tile on one of the just-committed cells, since that draft can no longer be placed; the
-// rack order is kept (Stage 17 #6).
+// rack order is kept.
 func (s *Store) resetConflictingBoardDrafts(ctx context.Context, gameID, actorID uuid.UUID, cells []DraftTile) error {
 	if len(cells) == 0 {
 		return nil