bench: add blob download read-ahead benchmark (read_prefetch sweep)

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

Cargo.toml

@@ -174,6 +174,14 @@ name = "bench_blob_manifest"
 path = "examples/bench_blob_manifest.rs"
 required-features = ["bench"]
 
+# Blob download read-ahead benchmark — sweeps the local backend's chunk prefetch
+# depth (read_prefetch / buffered(N)) under disk-bound vs network-bound consumers
+# and warm vs cold page cache. No Postgres needed.
+[[example]]
+name = "bench_blob_prefetch"
+path = "examples/bench_blob_prefetch.rs"
+required-features = ["bench"]
+
 # ACL owner-cache benchmark — owner query vs moka hit (needs the dev Postgres up).
 [[example]]
 name = "bench_owner_cache"