error: add is_disc_level_no_key classifier (re-exported)

Distinguishes a WHOLE-DISC key failure (E_NO_DISC_KEY / E_KEYDB_LOAD /
E_AACS_NO_KEYS -- every title fails identically) from a per-title skippable
stub. The engine's multi-title loop uses it to fail-fast on the first no-key
title instead of iterating all N. Additive.

src/lib.rs

@@ -155,7 +155,7 @@ pub use session::{
 // All fallible APIs return `Result<T, Error>`. `Error` is a typed enum with a
 // numeric `code()`; **no English text in the library** — applications map
 // codes to localized messages. See `error.rs` for the full taxonomy.
-pub use error::{Error, Result, is_halt, is_skippable_title_stub};
+pub use error::{Error, Result, is_disc_level_no_key, is_halt, is_skippable_title_stub};
 
 // ─── Cooperative cancellation ───────────────────────────────────────────────
 //