Adds `bench_blob_prefetch` (behind the `bench` feature) to measure the
local backend's chunk read-ahead depth — the `buffered(N)` read-ahead in
DedupService::stream_chunks fed by BlobStorageBackend::read_prefetch().
The bench reproduces the exact production reassembly combinator over a
real LocalBlobBackend (chunk files scattered across the 256 hash-prefix
dirs) and sweeps the prefetch depth under the two axes that decide whether
read-ahead helps on local disk:
- consumer speed: unthrottled (disk-bound) vs throttled@MB/s (network-bound)
- page cache: warm vs cold (posix_fadvise DONTNEED, Linux best-effort)
N=1 is current production; higher N is the candidate change. Lets us
verify gains/regressions empirically before changing read_prefetch(),
since the trait doc deliberately defaults local to 1.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JG5yYZ9s868mJwqT2Qz7ez
Claude