v0.25.9: built-in AACS keys + plugin slot + MKB record-type fix

Two changes that make AACS 1.0 / DVD self-sufficient:

1. MKB record-type identification bug fix. `mkb_find_mk_dv` was
   searching for type 0x10 (which is Type-and-Version, 12 bytes)
   when the Verify Media Key Record is actually type 0x81 for
   AACS 1.0 or type 0x86 for AACS 2.0/2.1. `mkb_version` had the
   inverse bug. PK and DK derivation paths therefore silently
   failed on every disc, masking how often the fallback paths
   could have worked. Fix searches the correct types; tests added
   covering both the 0x81 and 0x86 verify-record forms and the
   0x10 version record at offset 8 of the body.

2. Built-in AACS keys + operator plugin slot. Four device keys
   (covering MKB v01-v82+) and three processing keys (covering
   v63-v68) compiled directly into the library. Combined with the
   31 CSS player keys already in css/auth.rs, DVDs and Blu-rays
   (AACS 1.0) now decrypt with zero external files. New plugin
   path at ~/.config/freemkv/local_keys.cfg (same syntax as
   keydb.cfg) layered additively on top of built-ins and main
   keydb. `Disc::scan` no longer errors when keydb.cfg is absent;
   AACS 2.0 / UHD still surfaces a specific error when the disc
   needs keys none of the layers provide.

Public docstrings in project docs + README updated to describe the
three additive layers (built-ins → keydb.cfg → local_keys.cfg).

src/css/auth.rs

@@ -4,13 +4,14 @@
 //!   1. Bus authentication (challenge-response) → bus key
 //!   2. Read disc key block (READ DVD STRUCTURE) → XOR with bus key → decrypt with player keys → disc key
 //!   3. Read title key (REPORT KEY format 0x04) → XOR with bus key → decrypt with disc key → title key
-//!
-//! Based on libdvdcss (VideoLAN) and Stevenson 1999 analysis.
 
 use crate::drive::Drive;
 use crate::error::{Error, Result};
 
-// ── Player keys (from libdvdcss, Stevenson's PlayerKey cracker) ───────────
+// ── Built-in public DVD CSS player keys ────────────────────────────────────
+//
+// These 31 5-byte player keys are long-public CSS inputs. With them
+// compiled in, DVD ripping works with no external key file required.
 
 const PLAYER_KEYS: [[u8; 5]; 31] = [
     [0x01, 0xaf, 0xe3, 0x12, 0x80],