Add database format version header for safe migration #195

Source

PR #191 review thread: migration lacks format indicator

Problem

The migration fallback path in open_at_path has no format version indicator. Any file that fails GCM authentication (corrupted new-format data, bit rot, truncated writes) is assumed to be legacy AES-128-CTR. This means:

1. Corrupted new-format files go through legacy decryption with zero key/IV → garbage
2. String::from_utf8 fails with confusing error (no indication the real problem was corruption or wrong format)
3. If garbage happens to produce valid UTF-8 RON, database silently loads corrupt data

Fix

Add a 4-byte magic prefix to the file format:

const MAGIC_V2: [u8; 4] = *b"DMS2";  // AES-256-GCM format
const MAGIC_V1: [u8; 4] = *b"DMS1";  // Legacy AES-128-CTR format

// Write: prepend magic + nonce + ciphertext
// Read: check magic first, dispatch to correct decrypt path

New files get DMS2 prefix. Old files (DMS1 or no prefix) go through legacy migration. Unknown magic fails with clear error.

Relates: #191