dropIndex

Removes an index and deletes its posting files.

The base sidecars survive: they carry the key block and the present bitmap, which belong to the segment rather than to any index, and rebuilding them is exactly the cost the split into two files exists to avoid.

A file still being read is unmapped and deleted when the last reader closes, so this returns without waiting. Its id is never handed out again — a stale posting file left by a crash must not be readable as some later index's postings.