P2: per-chunk retry on transient timeout in fetch path

[sanity]

Problem

Matrix user feedback (ofansifkapital, 2026-05-07): git clone freenet::3GEERif5ihbf/freenet-core repeatedly times out at individual chunk GETs after 180s.

```
==> 6 bundle(s), 10.9 MiB total (~60s per chunk under load; up to 8 in parallel)
[3/6] downloading 3 chunks (2.7 MiB)
chunk 1/3
chunk 2/3
error: fetch ChunkedPack 8ff7da358a0c8a9f1c746b8e9e16d0c177d84210cff2706eabca09b1b2841522
Caused by:
0: GET chunk 2
1: timed out waiting for GET response after 180s
```

A single chunk timing out fails the entire clone. The next attempt redoes the whole thing (or, with #22 Phase 1 sidecar cache, just the chunks that didn't make it the first time — but that requires a previous successful attempt). In a degraded gateway window, no single attempt completes because at least one chunk in the manifest hits 180s on every attempt.

Approach

Add a per-chunk retry loop in chunked.rs's fetch path:

• N attempts (default 3) per chunk before bubbling failure.
• Exponential backoff between attempts (e.g. 5s, 15s, 45s).
• On retry, optionally drop and re-open the WS connection (current connection might be stuck in a bad state).
• Make N configurable via FREENET_GIT_CHUNK_RETRIES.

Combined with the #22 sidecar cache, this means:

1. First attempt: cache miss → network GET → 180s timeout → retry.
2. Retry: same cache key → still miss → network GET (maybe succeeds this time).
3. Second clone (after success): cache hit → instant.

Test plan

• Inject a stub fetcher that fails once then succeeds; assert retry succeeds.
• Inject a stub that always fails; assert it fails after N attempts with the right error.
• Assert backoff timing is bounded (don't sleep N*N total, just N+sum-of-backoffs).

[AI-assisted - Claude]