Add snappy streaming compression to WAL segments

[nakabonne]

Close #47

Why

WAL files grow rapidly under load . The root cause is that metric names are written uncompressed and repeated on every row, making WAL data highly compressible.

What

Introduce Snappy streaming compression to WAL segment files, targeting 2-3x file size reduction.

Changes:

• Writer chain: Insert snappy.Writer between bufio.Writer and os.File (bufio → snappy → file)
• Reader chain: Wrap segment file reads with snappy.NewReader for transparent decompression
• Helper methods: Extract initWriter / closeWriter to consolidate writer chain setup and teardown across
  newDiskWAL, punctuate, refresh, and removeAll
• Crash recovery: Add snappy.ErrCorrupt tolerance in readAll() — Snappy's frame format is self-synchronizing, so
  partial frames from a crash are safely skipped
• Test: Verify segment files contain the Snappy stream magic bytes (\xff\x06\x00\x00sNaPpY)

Backward compatibility

WAL segments are deleted on clean shutdown, so no migration is needed. The only edge case is a crash during
upgrade, where old uncompressed segments would fail to read — this is acceptable.

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 50.00000% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 60.44%. Comparing base (64a7605) to head (168b642).

Files with missing lines	Patch %	Lines
disk_wal.go	50.00%	7 Missing and 5 partials ⚠️
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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #70      +/-   ##
==========================================
- Coverage   60.45%   60.44%   -0.01%     
==========================================
  Files          19       19              
  Lines        1613     1625      +12     
==========================================
+ Hits          975      982       +7     
- Misses        502      505       +3     
- Partials      136      138       +2     

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