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/infrastructure/services/dedup_service.rs

@@ -1438,7 +1438,10 @@ impl DedupService {
         .map_err(|e| DomainError::internal_error("Dedup", format!("Manifest lookup: {}", e)))?;
 
         if let Some(chunk_hashes) = manifest {
-            // CDC file: stream chunks in order
+            // CDC file: stream chunks in order. Read-ahead depth is the
+            // backend's hint (1 for local disk, higher for remote object
+            // stores where overlapping fetches hide per-chunk latency).
+            let prefetch = self.backend.read_prefetch().max(1);
             let backend = self.backend.clone();
             let chunk_stream = stream::iter(chunk_hashes)
                 .map(move |chunk_hash| {
@@ -1450,7 +1453,7 @@ impl DedupService {
                             .map_err(|e| std::io::Error::other(e.to_string()))
                     }
                 })
-                .buffered(1)
+                .buffered(prefetch)
                 .try_flatten();
 
             Ok(Box::pin(chunk_stream))
@@ -1529,7 +1532,9 @@ impl DedupService {
                 }
             }
 
-            // Stream selected chunks with ranges
+            // Stream selected chunks with ranges. Read-ahead depth from the
+            // backend hint (local=1; remote overlaps fetches — see read_blob_stream).
+            let prefetch = self.backend.read_prefetch().max(1);
             let backend = self.backend.clone();
             let chunk_stream = stream::iter(selected)
                 .map(move |(chunk_hash, range_start, range_end)| {
@@ -1541,7 +1546,7 @@ impl DedupService {
                             .map_err(|e| std::io::Error::other(e.to_string()))
                     }
                 })
-                .buffered(1)
+                .buffered(prefetch)
                 .try_flatten();
 
             Ok(Box::pin(chunk_stream))