Tags: go-again/az

Add DecodeAllLimit and CI pipeline

Introduce a bounded decoding method to defend against decompression
bombs and implement a comprehensive GitHub Actions CI pipeline.

The new `DecodeAllLimit` prevents excessive memory allocation by
stopping decompression once the output exceeds a specified threshold.
It uses streaming reads for Zstd to ensure the limit is enforced without
materializing the entire frame first.

The CI pipeline adds:
- Multi-platform testing (Ubuntu, macOS, Windows) across multiple Go
  versions.
- Race detection and static analysis (go vet, staticcheck,
  golangci-lint).
- Cross-compilation checks for all supported GOOS/GOARCH targets.
- Modernization checks via gopls.

Add pooled Encoder/Decoder and agent documentation

Introduce poolable `Encoder` and `Decoder` for high-throughput usage,
reducing allocation churn by reusing internal lz4/zstd codecs. Added
`AGENTS.md` for library developers and `skills/` for library consumers.

Update documentation and benchmarks, add tests

Update README performance metrics and benchmark comparisons to reflect
newly measured values. Refactor internal string conversion logic for
improved safety and clean up generated code comments.

Replace custom LZ77+entropy engine with LZ4 and Zstandard

The custom LZ77/Huffman/FSE implementation is replaced with vendored
LZ4 (levels 1–2) and Zstandard (levels 3–5) algorithms copied from
github.com/pierrec/lz4/v4 and github.com/klauspost/compress. No
external imports are added; all algorithm code lives under internal/.

Level mapping:
-1  lz4 fast (hash-only, ~550 MB/s on text)
-2  lz4 HC depth 1024 (~290 MB/s)
-3  zstd SpeedDefault / level 6 (~350 MB/s)
-4  zstd SpeedBetterCompression / level 12 (~270 MB/s)
-5  zstd SpeedBestCompression / level 18 (~60 MB/s)

Wire format changes: levels 1–2 emit native LZ4 frames
(magic 0x184D2204), levels 3–5 emit native Zstandard frames
(magic 0xFD2FB528). The Reader auto-detects format from magic bytes,
so tar and standard lz4/zstd tools can decompress .az files directly.

The public API (Compress, Decompress, NewWriter, NewReader, options)
is unchanged. Module renamed to github.com/go-again/az.