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/mod.rs

@@ -31,9 +31,11 @@ pub struct Frame {
     pub data: Vec<u8>,
 }
 
-/// Convert 90kHz PTS to nanoseconds.
+/// Convert 90kHz PTS to nanoseconds (round to nearest).
 pub fn pts_to_ns(pts: i64) -> i64 {
-    pts * 100_000 / 9
+    // pts * 1_000_000_000 / 90_000 = pts * 100_000 / 9
+    // Add half-divisor for rounding: (pts * 100_000 + 4) / 9
+    (pts * 100_000 + 4) / 9
 }
 
 /// Trait for codec-specific elementary stream parsers.