v0.13.0: zero English in library + API hygiene + dead-code sweep

Audit pass against the project docs "no English text in library code" rule.
Found 9 call sites that violated the contract by stuffing English into
io::Error::new(kind, "…") or by abusing Error::DeviceNotFound { path }
as a free-form description field. Each is now a typed Error variant.

New variants and codes: ScsiInterfaceUnavailable (E1004), DeviceLocked
(E1005), IoKitPluginFailed (E1006), UnsupportedPlatform (E2003),
PlatformNotImplemented (E2004), MapfileInvalid (E6011), DiscUrlNotDirect
(E9009).

labels::apply() previously pushed Commentary/Descriptive/Score/IME and
" (Secondary)" English literals into AudioStream.label, leaking into
MKV titles + autorip UI. AudioStream now exposes structured `purpose:
LabelPurpose`, SubtitleStream `qualifier: LabelQualifier`. Callers
translate to localized text. label keeps codec-formatting only.

API hygiene: 11 mux/* modules dropped from `pub` to `pub(crate)` —
their *types* are still re-exported from lib.rs, but the modules were
leaking low-level EBML/TS/network primitives. Stream trait gets a real
rustdoc explaining read-vs-write split. lib.rs grouped re-exports into
documented sections. ScanOptions::with_keydb() removed (one-method-per-
action rule); use struct literal.

Dead-code sweep: removed lookahead.rs (orphan, never declared as mod),
tsreader.rs (TsDemuxReader unused), ebml::{write_int,read_vint,SEEK_*},
ts::{scan_first/last_pts,scan_duration,SCAN_HEAD/TAIL_SIZE,take/set_
remainder}, MkvMuxer codec_private_slots/filled fields and
fill_codec_private method (deferred-codecPrivate path never used since
the v0.10 PES rewrite). cargo clippy --all-targets -D warnings clean.

Tests: new error::tests for variant codes + Display "no English" guard +
io::ErrorKind mapping. 233 lib tests, all green (was 230).

Breaking: ScanOptions::with_keydb removed; mux/* modules pub(crate);
AudioStream and SubtitleStream gained required fields; UnsupportedDrive
{ product_revision: "Renesas not yet implemented" } no longer produced
(use PlatformNotImplemented).

src/disc/mod.rs

@@ -20,6 +20,11 @@ use crate::udf;
 
 use encrypt::HandshakeResult;
 
+// Re-export label classification enums alongside AudioStream / SubtitleStream
+// so the public surface keeps the structured metadata together. Callers map
+// these to display text in their own locale.
+pub use crate::labels::{LabelPurpose, LabelQualifier};
+
 // ─── Public types ───────────────────────────────────────────────────────────
 
 /// A scanned Blu-ray disc.
@@ -180,7 +185,11 @@ pub struct AudioStream {
     pub sample_rate: SampleRate,
     /// Whether this is a secondary stream (commentary)
     pub secondary: bool,
-    /// Extra label
+    /// Stream purpose (commentary / descriptive / score / IME / normal).
+    /// Callers translate this to display text in their own locale.
+    pub purpose: LabelPurpose,
+    /// Codec / variant text (e.g. "Dolby TrueHD 5.1", "(US)").
+    /// NEVER contains English purpose words — see `purpose` for that.
     pub label: String,
 }
 
@@ -195,6 +204,9 @@ pub struct SubtitleStream {
     pub language: String,
     /// Whether this is a forced subtitle
     pub forced: bool,
+    /// Subtitle qualifier (SDH / descriptive service / forced / none).
+    /// Callers translate this to display text in their own locale.
+    pub qualifier: LabelQualifier,
     /// Pre-formatted codec private data (e.g. VobSub .idx palette header)
     pub codec_data: Option<Vec<u8>>,
 }
@@ -889,13 +901,6 @@ pub struct ScanOptions {
 }
 
 impl ScanOptions {
-    /// Create options with a specific KEYDB path.
-    pub fn with_keydb(path: impl Into<std::path::PathBuf>) -> Self {
-        ScanOptions {
-            keydb_path: Some(path.into()),
-        }
-    }
-
     /// Resolve KEYDB path: explicit path first, then standard locations.
     fn resolve_keydb(&self) -> Option<std::path::PathBuf> {
         if let Some(p) = &self.keydb_path {