perf(dedup): backend-aware chunk read-ahead for CDC reassembly

read_blob_stream / read_blob_range_stream reassembled a CDC file by fetching
its chunks with `buffered(1)` — strictly sequential, so the next chunk's
backend fetch (a file `open` locally; a full request round-trip on S3/Azure)
only started after the current chunk was fully drained.

A benchmark of the exact pipeline (stream::iter(chunks).map(get).buffered(K)
.try_flatten()) showed a blind `buffered(4)` is the WRONG fix: on a local
disk it is neutral on a warm page cache and ~37% SLOWER cold, because
concurrent opens turn one sequential read into several competing random-I/O
streams over content-addressed (scattered) chunk files. The win is entirely
on remote backends, where per-chunk request latency dominates and overlapping
fetches hide it (≈ linear in K).

So the read-ahead depth is now a backend hint, not a constant:
- BlobStorageBackend::read_prefetch() default 1 (sequential; safe for local).
- S3 / Azure override to 8 (overlap GETs to hide TTFB).
- cached / encrypted / retry / migration delegate to the backend that serves
  the bytes.
- Both CDC read paths use `self.backend.read_prefetch().max(1)`.

Net: local backend unchanged (no regression); remote reassembly ~4-8x faster.
Ordered `buffered` (not buffer_unordered) keeps chunks in sequence.

Bench (per-chunk fetch-latency model): buffered(1)->(4)/(8) = x3.9 / x7.8
@1ms, x4.0 / x8.1 @5ms, x4.0 / x8.0 @20ms. Local warm: 230ms@1 vs 227ms@4
(noise); local cold: 425ms@1 vs 585ms@4 (why local stays at 1).

https://claude.ai/code/session_01DCszkkU11LYxMEUWr4setK

src/application/ports/blob_storage_ports.rs

@@ -126,4 +126,18 @@ pub trait BlobStorageBackend: Send + Sync + 'static {
     /// Only meaningful for local-filesystem backends.  Remote backends
     /// return `None`; callers that need a local file must stream + spool.
     fn local_blob_path(&self, hash: &str) -> Option<PathBuf>;
+
+    /// How many chunk fetches the CDC reader may run concurrently when
+    /// reassembling a file (`read_blob_stream`'s `buffered(N)` read-ahead).
+    ///
+    /// The default is **1** — sequential, because for a local disk concurrent
+    /// opens turn one sequential read into several competing random-I/O streams
+    /// over content-addressed (scattered) chunk files, which is neutral on a
+    /// warm page cache and *slower* cold. Remote backends (S3/Azure) override
+    /// this with a higher value: there the dominant cost is per-chunk request
+    /// latency, and overlapping fetches hides it (≈ N× faster reassembly).
+    /// Wrapping backends delegate to the backend that actually serves the bytes.
+    fn read_prefetch(&self) -> usize {
+        1
+    }
 }