mux/hevc: measure coding type from slice_type; reuse shared BitReader

Honest PictureInfo population for HEVC, on the same principle as H.264.

- Consolidate the bit reader: hevc.rs had its own BitReader (used by the SPS
  parser); h264 just gained one in startcode. Promote startcode's to the single
  shared pub(crate) reader (adds read_bits) and delete hevc's copy — one proven
  primitive, reused (SPS parse + both slice-type decoders).
- hevc: decode slice_type from the first coded slice's slice_segment_header
  (H.265 §7.3.6.1) → I/P/B (§7.4.7.1). The offset to slice_type depends on
  num_extra_slice_header_bits, which lives in the PPS — so we parse it from the
  ACTIVE PPS (§7.3.2.3) and only measure slice_type when that PPS is known.
  With no active PPS we decline rather than guess: coding stays None, honestly
  absent. Set coding = coding_type_only(...) and source = pes.source; field
  order (pic_struct SEI) is not decoded, so field_order() is honestly None.
- Tests: I/P/B from real slice headers, source carry, field-order absence, and
  the no-PPS honest-omission case.

src/mux/codec/startcode.rs

@@ -70,8 +70,17 @@ impl<'a> BitReader<'a> {
         Some(b as u32)
     }
 
+    /// Read `n` bits, MSB-first, into the low bits of a `u32`.
+    pub fn read_bits(&mut self, n: u32) -> Option<u32> {
+        let mut v = 0u32;
+        for _ in 0..n {
+            v = (v << 1) | self.read_bit()?;
+        }
+        Some(v)
+    }
+
     /// Skip `n` bits; `None` if that would run past the end.
-    pub fn skip_bits(&mut self, n: usize) -> Option<()> {
+    pub fn skip_bits(&mut self, n: u32) -> Option<()> {
         for _ in 0..n {
             self.read_bit()?;
         }