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/scsi/mod.rs

@@ -82,8 +82,8 @@ pub fn open(device: &Path) -> Result<Box<dyn ScsiTransport>> {
 
     #[cfg(not(any(target_os = "linux", target_os = "macos", target_os = "windows")))]
     {
-        Err(Error::DeviceNotFound {
-            path: format!("{}: unsupported platform", device.display()),
+        Err(Error::UnsupportedPlatform {
+            target: std::env::consts::OS.to_string(),
         })
     }
 }