Fix DASP close deadlock and kit checksum sign display

Use RLock instead of Lock for _close_lock so close() can call
_shutdown() without deadlocking — previously the session never
marked itself closed, causing infinite keepalive sends.

Mask kit checksums to unsigned 32-bit for hex display so negative
i4 values render as proper 8-digit hex (e.g. d3984c51 not -2c67b3af).

Tested against live device at 192.168.1.251 — full handshake,
schema read, and component tree load confirmed working.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sox.py

@@ -311,7 +311,7 @@ class KitPart:
         self.checksum = checksum
 
     def __repr__(self):
-        return f"{self.name}-{self.checksum:08x}"
+        return f"{self.name}-{self.checksum & 0xFFFFFFFF:08x}"
 
 
 class VersionInfo: