Captures three behaviors the restore hot path must exhibit but
currently doesn't, all under one test:
* Peak blob disk cache occupancy ≤ 1. Smart restore ordering should
drain every file referencing the currently-cached blob before
downloading the next one, so the sweeper can free each blob the
moment its file set is exhausted.
* Every remote blob fetched exactly once (counter on a wrapping
Storer). Already true today; the test pins it so neither future
cache-eviction nor reorder regressions can introduce
re-downloads.
* blobDiskCache.Get is never called during restore — chunk
extraction must go through ReadAt so we never read the whole
blob from disk to slice out a few KB. The 10 GB
photo-snapshot --debug output showed ~900 ms per cache-hit chunk
extract; ReadAt should bring that to sub-millisecond.
Adds Get/ReadAt call counters and a peak-Len tracker to
blobDiskCache, plus an internal restoreCacheObserver hook on Vaultik
so the test can capture the production cache instance without
exporting unexported types.
Currently fails with peak_len=3, get_calls=24, readat_calls=0. The
fix follows in subsequent commits.
sneak
a92b1a82ad