Locks the observable single-threaded ref_count accounting before the atomic-SQL
conversion: two enqueues of one (source,format) share a single asset and stack
ref_count to 2; release keeps the asset until the last holder then frees the
ready row; a non-holding (canceled) job's release is a no-op; the decrement
clamps at 0; an errored asset is kept at zero (only ready is freed). Green on the
current RMW code (5 pass) — the fix must keep every assertion.
peter