Repeatedly creating and dropping an index grows the database file by about 16 KB per cycle #209

Version: main (efacaf4).

What happens

10,000 cycles of CreateIndexAsync + DropIndexAsync on a typed collection grow the file from about 2 MB to about 166 MB. Found while testing #196; it happens on main the same way, so it isn't the metadata chain.

CollectionIndexManager.DropIndex frees the tree with FreeAllPages (CollectionIndexManager.cs:282-292). Two likely causes, not yet confirmed:

• pages with a WAL image are queued by fix(storage): don't reuse freed pages that still have a WAL image (#168) #169's deferred free until the next WAL truncation, and a tight loop without checkpoints never truncates;
• or some pages of the dropped tree aren't reached by CollectAllPages.

Repro sketch

for (int i = 0; i < 10_000; i++)
{
    await db.PeopleV2.CreateIndexAsync(p => p.Name, "idx_tmp");
    await db.PeopleV2.DropIndexAsync("idx_tmp");
}
// compare file size before and after; then CheckpointAsync() and compare again

Expected

After a checkpoint, the pages of the dropped indexes are reused and the file stops growing. If the growth disappears after a checkpoint, it's the deferred-free queue and only needs a size or count trigger for the checkpoint; if it doesn't, pages are leaking.