fix: Block checksum 32-bit overflow in _calc_chksum

The checksum accumulator was not masked to 32 bits during addition,
only at the end. With Python's arbitrary-precision integers, this
caused the sum to exceed 32 bits, producing wrong checksums for
blocks with large unsigned values (common in RDB headers).

Fix: mask with & 0xFFFFFFFF on each addition step.

This fixes RDB validation failing on valid Amiga disk images.

Author: metaneutrons

amitools/fs/block/Block.py

@@ -167,7 +167,7 @@ def _calc_chksum(self):
         chksum = 0
         for i in range(self.block_longs):
             if i != self.chk_loc:
-                chksum += self._get_long(i)
+                chksum = (chksum + self._get_long(i)) & 0xFFFFFFFF
         return (-chksum) & 0xFFFFFFFF
 
     def _get_timestamp(self, loc):