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/inttest/game_test.go

@@ -299,7 +299,7 @@ func TestResignWinnerAndStats(t *testing.T) {
 	}
 }
 
-// TestResignOnOpponentTurn checks the Stage 17 fix: a player can forfeit on the
+// TestResignOnOpponentTurn checks a player can forfeit on the
 // opponent's turn. Seat 0 plays (so it is seat 1's turn), then seat 0 resigns its own
 // seat while it is not its turn — no ErrNotYourTurn, the game ends, and seat 0 loses
 // despite leading on score.
@@ -464,7 +464,7 @@ func TestHintPolicy(t *testing.T) {
 	}
 }
 
-// TestGameVariant covers the edge's lightweight variant lookup (Stage 13): it returns the
+// TestGameVariant covers the edge's lightweight variant lookup: it returns the
 // created game's variant and ErrNotFound for an unknown id.
 func TestGameVariant(t *testing.T) {
 	ctx := context.Background()
@@ -619,7 +619,7 @@ func equalStrings(a, b []string) bool {
 	return true
 }
 
-// TestExportGCGRefusesActiveGame checks the Stage 8 finished-only gate: a GCG export
+// TestExportGCGRefusesActiveGame checks the finished-only gate: a GCG export
 // is allowed only once the game is over, so an active game leaks nothing mid-play.
 func TestExportGCGRefusesActiveGame(t *testing.T) {
 	ctx := context.Background()