docs: describe the generic Unlocker seam, drop in-tree firmware specifics

The docs still documented the old in-tree firmware unlocker: the MediaTek
MT1959 variant table, the READ BUFFER unlock CDB bytes, the profiles.json
schema (unlock_mode/unlock_buf_id/unlock_cdb), the platform/mt1959 driver
listings, and the 'why unlock is needed' handshake mechanism. None of that
lives in libfreemkv anymore — the core is firmware-clean and ships only the
pluggable Unlocker trait + registry (src/unlock.rs).

Rewrite drive-access, architecture, api-design, disc-to-rip, and the README
to describe only the generic Unlocker seam: the trait, register_unlocker, the
registry routing, and the host-cert fallback when no unlocker matches. Point
readers to the freemkv-unlock repo for concrete unlockers. No source change.

docs/disc-to-rip.md

@@ -10,14 +10,14 @@ Insert disc
     │
     ▼
 1. Open drive (drive/mod.rs)
-    │  INQUIRY → identify drive
-    │  Match bundled profile → chipset, unlock parameters
+    │  INQUIRY → identify drive (DriveId)
     │
     ▼
-2. Init drive (drive/mod.rs → platform/mt1959)
-    │  Firmware upload (if needed, 10s recovery wait)
-    │  Unlock → vendor-specific command activates raw read mode
-    │  Speed calibration → probe_disc()
+2. Init drive (drive/mod.rs → unlock seam)
+    │  Walk the registered-unlocker registry; first match unlocks the drive
+    │  (firmware/vendor handshakes are the unlocker's own business)
+    │  No match → drive untouched; host-cert AACS handshake carries the disc
+    │  Speed control → probe_disc()
     │
     ▼
 3. AACS handshake (aacs/handshake.rs) — optional