Disk cache: an edit that leaves the footage alone keeps its salt (#1926)

Session::bump took the disk cache's footage salt at every edit, which reads the size and time of
every footage file, each frame of an image sequence included. With three 5000-frame sequences a
caret move or a typed character in a text field took ~0.6 s on the UI thread (9 ms with the disk
cache off). The salt is now taken again only when the footage items changed, or when a project
is opened, footage is reloaded or the disk cache attached.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/engine/src/tests_sequence.rs

@@ -207,3 +207,31 @@ fn dpx_and_cineon_runs_import_as_sequences() {
     got.sort();
     assert_eq!(got, [("plate.[0001-0003].dpx".to_string(), 3), ("scan_[01-02].cin".to_string(), 2)], "{r}");
 }
+
+/// #1926: an edit that leaves the footage alone doesn't stamp every frame of its image sequences
+/// again for the disk cache's salt (with large sequences each keystroke in a text field took over
+/// half a second); Reload Footage does, and so does new footage.
+#[test]
+fn edits_keep_the_disk_salt_and_reload_stamps_the_files_again() {
+    let dir = folder(&RUN);
+    let cache = std::env::temp_dir().join(format!("ec-seq-salt-{}", std::process::id()));
+    let _ = std::fs::remove_dir_all(&cache);
+    let mut s = session();
+    s.execute_checked("prefs.set", json!({"values": {"disk.diskCacheFolder": cache.to_string_lossy(), "disk.diskCacheEnabled": true}})).unwrap();
+    s.execute("comp.new", json!({"name": "C", "width": 64, "height": 32, "frameRate": 30, "duration": 1})).unwrap();
+    let r = s.execute("file.import", json!({"paths": [at(&dir, "shot_0002.png")], "sequence": true, "addToComp": true})).unwrap();
+    let item = r["items"][0].clone();
+    let salt = s.refresh_disk_salt(false).expect("a disk cache");
+    // A frame rewritten in place: same footage settings, another size on disk.
+    std::fs::write(at(&dir, "shot_0004.png"), b"a new render of this frame").unwrap();
+    s.execute("layer.newText", json!({"text": "title"})).unwrap();
+    s.execute("layer.setText", json!({"text": "title 2"})).unwrap();
+    assert_eq!(s.refresh_disk_salt(false), Some(salt), "edits don't stamp the files again");
+    s.execute("file.reloadFootage", json!({"items": [item]})).unwrap();
+    let reloaded = s.refresh_disk_salt(false).unwrap();
+    assert_ne!(reloaded, salt, "Reload Footage stamps them again");
+    std::fs::write(at(&dir, "extra.png"), b"").unwrap();
+    s.execute("file.import", json!({"paths": [at(&dir, "extra.png")]})).unwrap();
+    assert_ne!(s.refresh_disk_salt(false), Some(reloaded), "new footage counts");
+    let _ = std::fs::remove_dir_all(&cache);
+}