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/labels/mod.rs

@@ -865,7 +865,7 @@ pub fn analyze(reader: &mut dyn SectorSource, udf: &UdfFs) -> LabelAnalysis {
 }
 
 /// Scan `/BDMV/PLAYLIST/*.mpls`, parse each, return a row per playlist
-/// with chapter count (mark_type ≤ 1) and total duration. Sorted by
+/// with chapter count (entry marks only) and total duration. Sorted by
 /// playlist filename. Skipped entries (read error, parse error, no
 /// marks) silently dropped — this is a diagnostic field, not a
 /// correctness-critical one.
@@ -890,7 +890,11 @@ fn collect_chapter_summary(reader: &mut dyn SectorSource, udf: &UdfFs) -> Vec<Ch
         let Ok(playlist) = crate::mpls::parse(&data) else {
             continue;
         };
-        let chapter_count = playlist.marks.iter().filter(|m| m.mark_type <= 1).count();
+        let chapter_count = playlist
+            .marks
+            .iter()
+            .filter(|m| m.is_chapter_mark())
+            .count();
         if chapter_count == 0 {
             continue;
         }