Implement SSH Compression support Phase 1

- compression.rs module with CompressionContext
- Compress/Decompress using flate2 (raw deflate, no zlib header)
- enable/disable/is_enabled methods
- compress/decompress with Sync flush
- Unit tests: disabled/enabled/roundtrip/supported

All tests pass.

markbase-core/src/ssh_server/mod.rs

@@ -4,6 +4,7 @@
 pub mod auth;
 pub mod channel;
 pub mod cipher;
+pub mod compression; // SSH Compression support (RFC 4253 §6.2)
 pub mod crypto;
 pub mod data_forwarder;
 pub mod kex;