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.

pkg/proto/push/v1/push.pb.go

@@ -84,7 +84,7 @@ type Event struct {
 	Kind    string                 `protobuf:"bytes,2,opt,name=kind,proto3" json:"kind,omitempty"`
 	Payload []byte                 `protobuf:"bytes,3,opt,name=payload,proto3" json:"payload,omitempty"`
 	EventId string                 `protobuf:"bytes,4,opt,name=event_id,json=eventId,proto3" json:"event_id,omitempty"`
-	// language routes an out-of-app push to a specific per-language bot (Stage 17): for a game
+	// language routes an out-of-app push to a specific per-language bot: for a game
 	// event it carries the game's language ("en"/"ru") so the notification comes from the game's
 	// bot rather than the recipient's last-login bot. Empty falls back to the recipient's service
 	// language at the gateway.

pkg/proto/push/v1/push.proto

@@ -33,7 +33,7 @@ message Event {
   string kind = 2;
   bytes payload = 3;
   string event_id = 4;
-  // language routes an out-of-app push to a specific per-language bot (Stage 17): for a game
+  // language routes an out-of-app push to a specific per-language bot: for a game
   // event it carries the game's language ("en"/"ru") so the notification comes from the game's
   // bot rather than the recipient's last-login bot. Empty falls back to the recipient's service
   // language at the gateway.