Verify key derivation is 100% correct

Breakthrough verification:
- Python computed keys match server actual keys EXACTLY
- Key derivation formula: HASH(K || H || X || session_id) verified
- All keys (encryption, MAC, IV) derived correctly
- Shared secret encoding (little-endian bytes) correct

Remaining issue:
- MAC verification fails despite correct key derivation
- Client must be computing different keys than server
- Need to compare client vs server actual key values

Next step: Wireshark comparison of OpenSSH client keys

markbase-core/src/ssh_server/kex_exchange.rs

@@ -106,7 +106,7 @@ impl KexExchangeHandler {
         
         info!("Exchange hash computed:");
         info!("  shared_secret[0] = {} (>=0x80? {})", shared_secret[0], shared_secret[0] >= 0x80);
-        info!("  exchange_hash (32 bytes): {:?}", &exchange_hash[..8]);
+        info!("  exchange_hash full (32 bytes): {:?}", exchange_hash);
         
         self.exchange_hash = Some(exchange_hash.clone());
         info!("Exchange hash saved for key derivation");