Implement cross-file persistent chunk cache

[Copilot]

Add a client-level, disk-backed chunk cache keyed by (xorbHash, absoluteChunkIndex) that is reused across all download methods and survives process restarts. No LRU or capacity limit.

Cache layer (download/persistent_cache.go)

• ChunkCache stores decoded chunks as files under <dir>/<xorbHash>/<chunkIndex>
• Atomic writes (temp + rename) for safe concurrent/cross-process access
• Get, Put, HasRange API

Prefetcher integration (download/prefetcher.go)

• Each prefetchEntry now tracks absolute chunk range (chunkStart/chunkEnd) derived from termFetches
• Hit path: tryLoadFromCache checks HasRange; on hit, creates a chunkCache backed by persistentRead — no network fetch, no decoding
• Miss path: saveToCache writes decoded chunks after LoadAll() in both single-range and multipart (stream) paths
• Best-effort writes; failures don't affect downloads

chunkCache extension (download/chunk_cache.go)

• Added persistentRead func(idx uint32) ([]byte, error) — when set, Chunk() bypasses the decoder/temp-file store entirely

Client wiring

• client.WithChunkCache(*download.ChunkCache) option, plumbed through download.WithChunkCache() to all four download entry points (DownloadFile, DownloadFileV1, DownloadFileV2, DownloadFiles)

Usage

cache, _ := download.NewChunkCache("/var/cache/xet/chunks")
c := client.NewClient(
    client.WithBaseURL("https://..."),
    client.WithChunkCache(cache),
)
// All subsequent downloads share and populate the same persistent cache
r, size, _ := c.DownloadFile(ctx, hash, nil)