feat(storage): add zstd replay compression

[DriftN2Forty]

Summary

• Switch new finalized binary replay archives to Zstd level 1 for both replay.bin timeline payloads and independently compressed chunk payloads.
• Add explicit timeline payload compression metadata via payloadCompression, while retaining magic-byte fallback for legacy manifests.
• Keep existing LZ4 timeline archives and chunk payload codec ID 1 readable; new chunk payloads write Zstd codec ID 2.
• Bump the replay compatibility floor to 1.5.0-alpha.12 so older alpha builds reject new Zstd archives before attempting LZ4 decode.
• Update binary format docs, manifest schema docs, README, architecture notes, and changelog.

Benchmark Context

Benchmarks were run with /replay debug compression against existing LZ4 archives using 3 runs and 1 warmup.

Sample BRAR-2026-06-16-22-16-45:

• Archive size: 17.82 MiB
• Timeline raw/current: 67.65 MiB / 17.01 MiB
• Timeline LZ4 default: 17.01 MiB, 25.14%, compress avg 116.47 ms, decompress avg 106.72 ms
• Timeline Zstd level 1: 7.12 MiB, 10.52%, compress avg 80.26 ms, decompress avg 58.42 ms
• Chunks raw/current: 2.24 MiB / 832.61 KiB across 105 payloads
• Chunk LZ4 default: 830.95 KiB, 36.20%, compress avg 303.45 ms, decompress avg 675.85 ms
• Chunk Zstd level 1: 557.80 KiB, 24.30%, compress avg 4.30 ms, decompress avg 1.85 ms

Sample BRAR-2026-06-17-21-52-51:

• Archive size: 73.81 MiB
• Timeline raw/current: 279.12 MiB / 73.74 MiB
• Timeline LZ4 default: 73.74 MiB, 26.42%, compress avg 343.31 ms, decompress avg 231.77 ms
• Timeline Zstd level 1: 6.25 MiB, 2.24%, compress avg 398.90 ms, decompress avg 505.22 ms
• Chunks raw/current: 198.20 KiB / 73.42 KiB across 9 payloads
• Chunk LZ4 default: 73.26 KiB, 36.96%, compress avg 298.63 ms, decompress avg 298.46 ms
• Chunk Zstd level 1: 49.46 KiB, 24.95%, compress avg 0.49 ms, decompress avg 0.35 ms

These results favor Zstd level 1 as the default because it gives substantially smaller timelines and chunk payloads while keeping compression cost acceptable for new saves.

Compatibility

• New archives include payloadCompression: "zstd".
• Archives missing payloadCompression still decode by deterministic frame magic detection.
• LZ4 frame magic: 04 22 4D 18.
• Zstd frame magic: 28 B5 2F FD.
• Existing chunk codec ID 1 remains LZ4_FRAME.
• New chunk codec ID 2 is ZSTD.
• The .br ZIP container layout is unchanged and chunk payloads remain selectively decompressible.

Tests

• mvn test
• Result: 550 tests, 0 failures, 0 errors, 0 skipped

Added/updated regression coverage for:

• Zstd timeline archive decoding.
• Existing LZ4 timeline archive decoding.
• Missing manifest compression field fallback via magic bytes.
• Unsupported timeline compression errors.
• New Zstd chunk codec ID usage and decoding.
• Existing LZ4 chunk payload decoding.
• Unsupported chunk codec ID failures.