Fix all PES pipeline audit findings (20 issues)

Critical:
- C1: PES serialize validates track < 256 and data < 4GB
- C2: PES deserialize caps frame size at 256MB (OOM protection)
- C3: TsMuxer stuffing uses static buffer, no per-packet alloc
- C4: PES length uses unbounded (0x0000) for audio >65535 bytes
- C6: TsDemuxer validates AF length <= 183

Warning:
- W1: parse_timestamp validates marker bits, returns Option
- W2: PES header data_start clamped to data.len()
- W3: TsMuxer PTS conversion uses saturating_mul, rejects negative
- W4: AC3/DTS replace debug_assert with runtime bounds check
- W6: MKV block_vint handles 3-4 byte VINTs
- W7: meta.rs to_title() uses unwrap_or fallbacks instead of panic
- W8: MKV reader skips frames for non-existent tracks
- W9: DVD PTS uses higher-precision conversion (1e9/90000)
- FMKV read_header caps JSON at 10MB
- PAT section_len underflow guard

Suggestion:
- S2: TsMuxer uses static STUFF_FF buffer
- S3: HEVC parser single-pass NAL scan (was duplicated)
- S4: TsDemuxer caps remainder at one packet
- S5: PTS 90kHz→ns uses round-to-nearest

src/mux/codec/dts.rs

@@ -82,7 +82,9 @@ pub fn find_dts_hd_ext_sync(data: &[u8]) -> Option<usize> {
 /// The size field is at bytes 6-8 of the extension:
 ///   ((ext[6] & 0x1F) << 11) | (ext[7] << 3) | (ext[8] >> 5) + 1
 pub fn dts_hd_ext_frame_size(ext: &[u8]) -> usize {
-    debug_assert!(ext.len() >= 9);
+    if ext.len() < 9 {
+        return 0;
+    }
     let raw =
         ((ext[6] as usize & 0x1F) << 11) | ((ext[7] as usize) << 3) | ((ext[8] as usize) >> 5);
     raw + 1