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/adminconsole/views.go

@@ -60,7 +60,7 @@ type UsersView struct {
 
 // UserRow is one account row in the list. MoveMin/Avg/Max are the account's
 // pre-formatted move-duration summary (empty when it has no timed move);
-// FlaggedHighRate marks the soft high-rate badge (R3).
+// FlaggedHighRate marks the soft high-rate badge.
 type UserRow struct {
 	ID              string
 	DisplayName     string
@@ -111,10 +111,10 @@ type UserDetailView struct {
 	NotificationsInAppOnly bool
 	PaidAccount            bool
 	// MergedInto is the primary account id when this account has been retired by a
-	// merge (Stage 11), or empty for a live account.
+	// merge, or empty for a live account.
 	MergedInto string
 	// FlaggedHighRateAt is the pre-formatted soft high-rate marker timestamp,
-	// empty for an unflagged account; the card shows it with the Clear action (R3).
+	// empty for an unflagged account; the card shows it with the Clear action.
 	FlaggedHighRateAt string
 	HintBalance       int
 	CreatedAt         string