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/session/cache.go

@@ -97,8 +97,8 @@ func (c *Cache) Remove(tokenHash string) {
 }
 
 // RemoveByAccount evicts every cached session belonging to accountID. The
-// account-merge flow uses it to drop a retired secondary account's sessions
-// (Stage 11); a linear scan is adequate at the cache's size.
+// account-merge flow uses it to drop a retired secondary account's sessions;
+// a linear scan is adequate at the cache's size.
 func (c *Cache) RemoveByAccount(accountID uuid.UUID) {
 	if c == nil {
 		return