Deduplicate PID/unpad helpers and generalize the Topaz path fix

Move the copies of unpad, crc32, checksumPid and pidFromSerial that were
scattered across the plugin into utilities.py, and add a shared safe_join
that generalizes the Topaz extraction path-traversal fix.

- unpad: adobekey, ineptepub and ineptpdf import the shared helper. The
  four bare-script key tools keep their local copies since they run
  without a package context.
- checksumPid is type-preserving (bytes for kgenpids/kindlepid, str for
  mobidedrm) so every call site keeps its exact behavior.
- kgenpids falls back to an absolute import because it is imported
  top-level by the worker modules.
- topazextract.extractFiles uses safe_join; genbook is unchanged as it
  only handles already-sanitized names.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

DeDRM_plugin/mobidedrm.py

@@ -78,14 +78,13 @@ __version__ = "1.1"
 import sys
 import os
 import struct
-import binascii
 
 
 #@@CALIBRE_COMPAT_CODE@@
 
 
 from .alfcrypto import Pukall_Cipher
-from .utilities import SafeUnbuffered
+from .utilities import SafeUnbuffered, checksumPid
 from .argv_utils import unicode_argv
 
 
@@ -105,27 +104,6 @@ def PC1(key, src, decryption=True):
     except: 
         raise
 
-letters = b'ABCDEFGHIJKLMNPQRSTUVWXYZ123456789'
-
-def crc32(s):
-    return (~binascii.crc32(s,-1))&0xFFFFFFFF
-
-def checksumPid(s):
-
-    s = s.encode()
-
-
-    crc = crc32(s)
-    crc = crc ^ (crc >> 16)
-    res = s
-    l = len(letters)
-    for i in (0,1):
-        b = crc & 0xff
-        pos = (b // l) ^ (b % l)
-        res += bytes(bytearray([letters[pos%l]]))
-        crc >>= 8
-    return res.decode()
-
 # expects bytearray
 def getSizeOfTrailingDataEntries(ptr, size, flags):
     def getSizeOfTrailingDataEntry(ptr, size):