Add HASHAB signing to iTunesCDB header + fix sync_itunescdb

- Set hashing_scheme=3 at offset 0x30 (iPod Nano 7G requirement)
- Compute and write HASHAB signature at offset 0xAB
- Import compute_hashab from hashab module
- iTunesCDB.ext creation restored (was removed accidentally)
- 0-byte iTunesDB placeholder (firmware expects this file)

src/ipod_nano7_db.py

@@ -1200,30 +1200,59 @@ class Nano7Database:
         """
         import zlib
         import re
+        from hashab import compute_hashab
         
         # Read current Library.itdb
         with open(self.sqlite_path, "rb") as f:
             lib_data = f.read()
         
-        # Compute new SHA1 hash
+        # Compute new SHA1 hash of Library.itdb for iTunesCDB.ext
         new_hash = hashlib.sha1(lib_data).hexdigest()
         
         compressed = zlib.compress(lib_data)
         total_size = 244 + len(compressed)
 
-        # Build proper mhfd header
+        # Build mhfd header
         header = bytearray(244)
         header[0:4] = b"mhfd"
         struct.pack_into("<I", header, 4, 244)
         struct.pack_into("<I", header, 8, total_size)
         struct.pack_into("<I", header, 12, 0x00020002)
 
+        # Set hashing_scheme = 3 (HASHAB) at offset 0x30
+        struct.pack_into("<H", header, 0x30, 3)
+
+        # Build full iTunesCDB (header + compressed payload)
+        itdb_data = bytes(header) + compressed
+
+        # Compute HASHAB signature and write it at offset 0xAB
+        if self.firewire_id and len(self.firewire_id) >= 8:
+            try:
+                signature = compute_hashab(hashlib.sha1(itdb_data).digest(), self.firewire_id)
+                header[0xAB:0xAB + 57] = signature
+                itdb_data = bytes(header) + compressed
+                logger.info("HASHAB signature written to iTunesCDB header")
+            except Exception as e:
+                logger.warning("HASHAB signing failed for iTunesCDB: %s", e)
+
         # Write new iTunesCDB
         with open(self.cdb_path, "wb") as f:
-            f.write(bytes(header) + compressed)
+            f.write(itdb_data)
 
         logger.info(f"Synced iTunesCDB: hash={new_hash}, compressed={len(compressed)} bytes")
 
+        # Update hash in iTunesCDB.ext
+        if not os.path.exists(self.cdb_ext_path):
+            ext_content = f"[iTunesCDB]\nitunesdb_hash={new_hash}\n"
+        else:
+            with open(self.cdb_ext_path, "r") as f:
+                ext_content = f.read()
+        new_ext_content = re.sub(r"itunesdb_hash=\w+", f"itunesdb_hash={new_hash}", ext_content)
+        with open(self.cdb_ext_path, "w") as f:
+            f.write(new_ext_content)
+
+        logger.info(f"Updated iTunesCDB.ext: hash={new_hash}")
+
     def sync_locations_cbk(self):
         """Regenerate Locations.itdb.cbk with HASHAB-signed block checksums.