Reject a short READ CAPACITY reply, and count only entry marks as chapters

Two cases of the same shape: one policy implemented twice, with only one
copy hardened.

Disc::read_capacity decoded buf[0..4] from READ CAPACITY (10) without
checking that the transport actually delivered four bytes, even though its
comment claims to mirror decode_read_capacity — which has exactly that
check, and documents why. A drive answering GOOD with an empty data phase
leaves the buffer zeroed, so last_lba decodes to 0 and the probe reports a
one-sector disc instead of an error. It now calls the shared decoder rather
than re-deriving it.

collect_chapter_summary filtered chapters on mark_type <= 1, counting the
reserved type 0. PlaylistMark's own doc says filters must test == 1, and
disc/bluray.rs did; the labels path did not, inflating the public
chapter_count and letting a playlist whose only marks are reserved pass the
chapter_count == 0 skip. Both sites now share PlaylistMark::is_chapter_mark
so the copies cannot drift again.

Both fixes were confirmed red before green.

src/disc/bluray.rs

@@ -298,7 +298,7 @@ impl Disc {
         let chapters: Vec<Chapter> = parsed
             .marks
             .iter()
-            .filter(|m| m.mark_type == 1)
+            .filter(|m| m.is_chapter_mark())
             .filter_map(|m| {
                 let pi_idx = m.play_item_ref as usize;
                 let pi = parsed.play_items.get(pi_idx)?;