Motivation:

The idle cleaner read a channel's idle timestamp after deciding it had expired, so a lease and re-offer in that window closed a healthy keep-alive on a stale verdict (#2284). Hoisting that read alone leaves a second race: claim and validation stay two steps, so the cleaner can briefly own a generation it never evaluated while a racing poll drops the channel from the pool entirely.

Modification:

IdleState packs ownership, a transfer-in-progress flag and a generation counter into one atomic word, and the cleaner claims with a single CAS against the exact generation its verdict came from.
offer() now transfers ownership rather than stamping unconditionally, and flushPartitions claimsbefore closing so it never closes a channel another caller owns.

Result:
Fixes #2284