Repository.get_many: coalesce pack reads and prefetch — remote extract/mount/check are pure-latency-bound

From a global code review of the borg2 codebase (2026-08).

### Problem

With the borg1 RPC gone, all object reads go through borgstore, and `Repository.get_many()` is strictly sequential: one `store.load(offset, size)` round trip per object (`src/borg/repository.py`, `get_many` → `PackReader.read`). `PackReader.iter_headers` likewise does one short range read per object header.

borg 1.x's remote protocol pipelined gets with readahead; borg2 currently has no equivalent, so on high-latency backends (rest over WAN, s3, sftp, rclone) restore/mount/check throughput is dominated by RTT, not bandwidth: restoring 100k objects at 30 ms RTT is ~50 minutes of pure waiting, regardless of object size.

### Proposed direction

All transparent behind the existing `get_many()` generator contract:

1. **Group by pack**: the chunk index already knows `(pack_id, offset, size)` for every requested id. Sort requests per pack by offset and **coalesce adjacent/nearby ranges** into a single ranged `store.load` per cluster (objects written together during create tend to be read together during extract, so locality is usually excellent).
2. **Prefetch across clusters**: a small bounded pool (or async loop) keeping N loads in flight, yielding results in request order.
3. Reuse the same machinery for `check`'s header walks (`iter_headers` currently pays one RTT per object).

Related: #1678 (repeated-chunk cache, merged), #5110 (FUSE fetch path), #37 (parallelism in general), #9988 (write-side async pack upload). This issue is the read-side counterpart and is probably the single biggest performance win available for remote repos.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Start with src/borg/repository.py at Repository.get_many(), then trace PackReader.read, PackReader.iter_headers, and the existing store.load calls. Review the related issues for constraints before choosing an approach. Done means remote reads are coalesced and bounded in flight while preserving the get_many() request order, with header walks benefiting as well.

Written by the indexing model from the issue text.