1.2.0: unify hex parsing (one parser) + robust encrypted-unit sampling

- 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.

src/lib.rs

@@ -111,6 +111,7 @@ pub mod dvdnav;
 pub mod error;
 pub mod event;
 pub mod halt;
+pub mod hex;
 pub(crate) mod identity;
 pub(crate) mod ifo;
 pub mod io;