Recover kit checksums from a truncated Sox schema reply

The device builds the schema and version replies in a fixed 512-byte
packet and drops whatever does not fit. Read strings defensively, recover
a cut-off checksum from the one local manifest matching its prefix, and
fail with a clear message when the kit list cannot be recovered.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

schema.py

@@ -224,9 +224,32 @@ def find_manifest_path(kit_name, checksum):
     return None
 
 
+def resolve_truncated_checksum(kit_name, prefix):
+    """The checksum of the one local manifest whose leading bytes match ``prefix``.
+
+    Used when a device's schema reply cut a kit's checksum short. Returns the
+    checksum as a signed 32-bit int (as read off the wire), or None when no
+    manifest, or more than one, matches.
+    """
+    manifest_dir = os.path.join(app_state.SEDONA_HOME, "manifests", kit_name)
+    if not os.path.isdir(manifest_dir):
+        return None
+    want = prefix.hex()
+    matches = set()
+    for f in os.listdir(manifest_dir):
+        if f.startswith(f"{kit_name}-") and f.endswith(".xml"):
+            hex_part = f[len(kit_name) + 1:-4].zfill(8).lower()
+            if hex_part.startswith(want):
+                matches.add(hex_part)
+    if len(matches) != 1:
+        return None
+    value = int(matches.pop(), 16)
+    return value - (1 << 32) if value & 0x80000000 else value
+
+
 def find_manifest(kit_name, checksum):
     """Returns the parsed ET root of a kit manifest, or None."""
-    checksum_hex = f"{checksum:08x}" if isinstance(checksum, int) else checksum
+    checksum_hex = f"{checksum & 0xFFFFFFFF:08x}" if isinstance(checksum, int) else checksum
     path = find_manifest_path(kit_name, checksum_hex)
     if path is None:
         return None