Adds parse_hex_u16/u32/u8 and exposes strip_hex_prefix so callers stop hand-rolling
from_str_radix(trim_start_matches("0x")) — a case-sensitive strip that this module
exists to prevent. disc::aacs_disc_hash now uses strip_hex_prefix.
- 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.
Matthew Jackson