Say when a title set is dropped from a DVD scan

parse_vmg skipped any title set whose parse failed, with no log and no
counter. A real disc enumerated 38 titles from one image and 10 from
another, and the 28 discarded failures were invisible — the symptom read
as a scan difference rather than as dropped reads, which is most of why
it took so long to localise.

Behaviour is unchanged: a disc may legitimately carry placeholder TT_SRPT
entries, so one failure is still not fatal. It now warns per skip with the
title set and the error, and once at the end with kept-versus-declared.

Also adds an opt-in diagnostic that reads every VTS IFO from an image, to
separate a read failure from a parse failure. It reports all 13 sets
reading identically from both a CSS image and its decrypted copy, which is
what proves the 38-to-10 loss is downstream of these reads.

src/dirimage/tests.rs

@@ -824,3 +824,29 @@ fn dump_titles_for_an_image() {
         }
     }
 }
+
+/// Diagnostic (opt-in): can each VTS IFO be READ from an image?
+///
+/// `parse_vmg` skips a title set whose `parse_vts` fails, and `parse_vts`
+/// begins by reading `/VIDEO_TS/VTS_nn_0.IFO`. This isolates the read.
+#[test]
+#[ignore = "diagnostic: needs FMKV_IMAGE"]
+fn dump_vts_ifo_reads_for_an_image() {
+    let Ok(path) = std::env::var("FMKV_IMAGE") else {
+        return;
+    };
+    let mut img =
+        crate::io::file_sector_source::FileSectorSource::open(std::path::Path::new(&path))
+            .expect("open");
+    let fs = udf::read_filesystem(&mut img).expect("udf");
+    for n in 1..=20u32 {
+        let p = format!("/VIDEO_TS/VTS_{n:02}_0.IFO");
+        match fs.read_file(&mut img, &p) {
+            Ok(b) => {
+                let magic = String::from_utf8_lossy(&b[..12.min(b.len())]).to_string();
+                println!("VTS {n:02}: read ok, {} bytes, magic={magic:?}", b.len());
+            }
+            Err(e) => println!("VTS {n:02}: READ FAILED: {e}"),
+        }
+    }
+}

src/ifo.rs

@@ -421,16 +421,40 @@ pub fn parse_vmg(reader: &mut dyn SectorSource, udf: &UdfFs) -> Result<DvdInfo>
 
     // Parse each VTS IFO
     let mut title_sets = Vec::new();
+    let mut skipped = 0usize;
     for (&vts_number, titles_info) in &title_set_map {
         match parse_vts(reader, udf, vts_number, titles_info) {
             Ok(ts) => title_sets.push(ts),
-            Err(_) => {
-                // Skip unreadable title sets — some DVDs have placeholder entries.
+            Err(e) => {
+                // Some discs carry placeholder TT_SRPT entries for title sets
+                // that are not really there, so one failure is not fatal. But
+                // the failure must not be INVISIBLE: every skipped set is a
+                // title the user will never see, and swallowing the reason made
+                // a disc that enumerated 38 titles from one image and 10 from
+                // another look like a scan difference rather than 28 dropped
+                // reads.
+                skipped += 1;
+                tracing::warn!(
+                    target: "freemkv::scan",
+                    vts = vts_number,
+                    titles = titles_info.len(),
+                    error = %e,
+                    "title set could not be parsed; its titles are omitted"
+                );
                 continue;
             }
         }
     }
 
+    if skipped > 0 {
+        tracing::warn!(
+            target: "freemkv::scan",
+            skipped,
+            kept = title_sets.len(),
+            declared = title_set_map.len(),
+            "some title sets were omitted from the scan"
+        );
+    }
     Ok(DvdInfo { title_sets })
 }