cleanup orphan detection reads via topology

Swap ListOrphanedSessionStates from NewGitStore (v1-pinned) to NewCommittedReadStore (topology-aware) so v1.1 mode reads orphan candidates from the configured mirror ref.

A failed mirror advance under v1.1 could cause a false-orphan flag; this trade-off is intentional. v1.1 is internal-only until topology inversion lands.

Adds a TDD test that enables v1.1 with no mirror ref present and asserts that a session whose checkpoint exists only on v1 is flagged as orphaned.