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).
