perf: Implement Adaptive Compression (zstd with file-type heuristic & multi-threading)

[kavix]

🎯 Goal

Upgrade Eko's blob compression from standard gzip to zstd with adaptive compression levels and file-type detection.

💡 Why

• zstd offers 3x faster decompression speed than gzip while achieving higher compression ratios (4–6x on text).
• Already compressed files (e.g. .png, .zip, .mp4, .gz) waste CPU when compressed again.
• Text files (.go, .js, .json, .md) compress extremely well.

🛠️ Implementation Steps

1. Integrate github.com/klauspost/compress/zstd in internal/objects/store.go.
2. Add a file extension heuristic check: skip compression entirely for binary media/archive extensions (.png, .jpg, .zip, .tar, .pdf).
3. Use multi-threaded zstd encoding for blobs larger than 1MB.
4. Benchmark save/restore speeds and disk size across text vs binary test suites.