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>

main.py

@@ -1961,7 +1961,7 @@ def _load_live_schema(ds):
     app_state.final_components_registry.clear()
     if ds.sox_client and ds.sox_client.schema_parts:
         for part in ds.sox_client.schema_parts:
-            app_state.active_schema_kits[part.name] = f"{part.checksum:08x}"
+            app_state.active_schema_kits[part.name] = f"{part.checksum & 0xFFFFFFFF:08x}"
         try:
             schema.load_schema_kits_and_manifests()
         except Exception: