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/mux/ebml.rs

@@ -87,11 +87,6 @@ pub fn write_uint(w: &mut impl Write, id: u32, val: u64) -> io::Result<()> {
     }
 }
 
-/// Write a complete EBML signed integer element.
-pub fn write_int(w: &mut impl Write, id: u32, val: i64) -> io::Result<()> {
-    write_uint(w, id, val as u64)
-}
-
 /// Write a complete EBML float element (8-byte double).
 pub fn write_float(w: &mut impl Write, id: u32, val: f64) -> io::Result<()> {
     write_id(w, id)?;
@@ -309,22 +304,6 @@ pub fn read_binary_val(r: &mut impl Read, len: usize) -> io::Result<Vec<u8>> {
     Ok(buf)
 }
 
-/// Read a VINT (track number) from a SimpleBlock. Returns (value, bytes_consumed).
-pub fn read_vint(r: &mut impl Read) -> io::Result<(u64, usize)> {
-    let mut first = [0u8; 1];
-    r.read_exact(&mut first)?;
-    let b0 = first[0];
-    if b0 & 0x80 != 0 {
-        return Ok(((b0 & 0x7F) as u64, 1));
-    }
-    if b0 & 0x40 != 0 {
-        let mut b = [0u8; 1];
-        r.read_exact(&mut b)?;
-        return Ok(((((b0 & 0x3F) as u64) << 8) | b[0] as u64, 2));
-    }
-    Err(crate::error::Error::MkvInvalid.into())
-}
-
 // ============================================================
 // Matroska Element IDs
 // ============================================================
@@ -342,12 +321,6 @@ pub const EBML_DOC_TYPE_READ_VERSION: u32 = 0x4285;
 // Segment
 pub const SEGMENT: u32 = 0x1853_8067;
 
-// Seek Head
-pub const SEEK_HEAD: u32 = 0x114D_9B74;
-pub const SEEK: u32 = 0x4DBB;
-pub const SEEK_ID: u32 = 0x53AB;
-pub const SEEK_POSITION: u32 = 0x53AC;
-
 // Segment Info
 pub const INFO: u32 = 0x1549_A966;
 pub const TIMESTAMP_SCALE: u32 = 0x2A_D7B1;