feat(compression): add reversible LZ77 compression

[google-labs-jules]

Summary

• add LZ77 (1977) compression and decompression
• replace the previously known-broken decompression implementation
• support overlapping sliding-window back-references
• validate malformed token lengths, literal offsets, and invalid back-references
• integrate LZ77 into Eddy's chronological Compression menu
• reuse the shared Compression UI registration so LZ77 always exposes both Compress and Decompress
• extend the common round-trip suite to include LZ77
• add dedicated overlap, arbitrary-byte, and malformed-input regressions
• document LZ77 in the README

Reversibility invariant

Every compression algorithm exposed by Eddy must provide and validate both directions:

decompress(compress(input)) == input

The same principle applies to encryption algorithms: every encrypt operation must have its matching decrypt operation.

Validation

• round-trip validated for repetitive/overlapping data
• round-trip validated for all 256 byte values
• random binary round-trips validated during implementation
• malformed LZ77 streams are rejected
• repository CI runs the Gradle build/test suite

Closes #2

[google-labs-jules]

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