Pre-flight check_cache_limit reads the current total_size_bytes and
returns an informative error if the cache is already at or over the
configured limit. Wired into v2_cache_track_for_offline (single) and
v2_cache_tracks_batch_for_offline (batch). Re-download paths are not
gated since they target tracks already counted toward the total. The
check intentionally does not predict the new track's size; preventing
cache from growing further once at the limit is enough for v1.
cc
vicrodh
1b07036454