FsStore previously CBOR-decoded all .bin nodes into memory at startup,
making cold-open O(n) in time and memory. Now it scans only filenames
into a Set<Hash> at init and reads/decodes nodes from disk on first
get(). has() and listAll() use the filename set; put() write-throughs
to cache; delete() clears cache and disk. Index/meta migration still
performs a one-time scan when _index/ is missing.
Adds 12 new tests (L1-L12) covering startup-no-decode, lazy get,
filename-based has/listAll, write-through put, delete cleanup, list
operations, and migration/bootstrap regression. All existing tests
pass unchanged.
Fixes#85
xiaoju