- One workspace hex parser (libfreemkv::hex): the keydb / online / mapfile
parsers had divergent prefix handling (0x vs 0X vs none) — a key written with
a prefix one parser didn't expect was silently dropped. All three now call it.
- read_encrypted_units probes several points spread across each extent instead
of only the midpoint-forward window, so a late-starting / sparse encrypted
body still yields samples (empty samples make decrypt_with skip wrong-key
validation). A read error at one probe no longer abandons the extent.
