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/h264.rs

@@ -180,6 +180,9 @@ pub fn find_start_code(data: &[u8], from: usize) -> Option<usize> {
     if data.len() < from + 3 {
         return None;
     }
+    // Range excludes last 2 bytes since we read 3 bytes at each position.
+    // data.len()-2 as exclusive upper bound means last checked index is data.len()-3,
+    // which accesses data[len-3], data[len-2], data[len-1] — all valid.
     (from..data.len() - 2).find(|&i| data[i] == 0x00 && data[i + 1] == 0x00 && data[i + 2] == 0x01)
 }