feat: add structured debug logging for troubleshooting

[coreyleavitt]

Summary

• Add structured debug logging throughout kdbxtool for troubleshooting
• Follow Python library best practices (NullHandler, logging.getLogger(__name__))
• Never log sensitive data (passwords, keys, decrypted content)

Changes

File	Logging Added
__init__.py	NullHandler to prevent warnings
parsing/header.py	Version, cipher, compression, KDF params
parsing/kdbx4.py	Decryption/encryption progress, HMAC verification
parsing/kdbx3.py	Decryption progress, block verification
security/kdf.py	KDF start/complete, composite key (booleans only)
security/crypto.py	Cipher initialization
security/keyfile.py	Format detection, keyfile creation
security/yubikey.py	Device count, challenge-response
database.py	Open/save/create/reload, entry/group counts

Usage

import logging
logging.basicConfig(level=logging.DEBUG)

from kdbxtool import Database
db = Database.open("test.kdbx", password="test")

Test plan

• All 706 tests pass
• Verified no sensitive data is logged (passwords, keys, salts, decrypted content)
• NullHandler prevents warnings when logging not configured

Closes #29