carry of #27737: distinguish NEXT nightly update identity
Original purpose: distinguish the cmux NEXT nightly update track from classic NIGHTLY while preserving test-feed, managed-channel, labels, status/probe, and channel-switch behavior. This carry merges current feat-cmux-next and preserves the original change.
No capture: protocol-only updater behavior, no visible UI.
Fresh exact-head independent review: Astra SHIP at a442e9c15e7ad690522f55f23183f056a9469d2b. The fix keeps the internal nightly-next track in updates.track while cmux version exposes the established public nightly channel; regression coverage asserts both values.
Fleet validation: focused UpdateTrackMatrixTests, TipAppTrackTests, and ReleasePushListenerTests submitted at a442e9c15e7ad690522f55f23183f056a9469d2b (run pending).
Agent session: agent session