Keyframes: mute keyframes without deleting them (#1894)

Add a muted flag to Keyframe. Muted keys stay in the Timeline and Graph Editor (dimmed, selectable and movable) but evaluation skips them, so the active neighbours interpolate as if they were not there; auto-Bezier and eases use the active neighbours. When every key is muted the property falls back to its static value.\n\nkeys.setMuted {muted} mutes or unmutes the selected keys in one undo step. The Timeline context menu offers Mute/Unmute Keyframes and the Animation menu has Mute Keyframes. Muted keys have no Bezier handles and motion-path points are dimmed.\n\nLottie export omits muted keys (with a warning) because Lottie has no mute state; scripting keeps seeing them in numKeys/key().\n\nTests: keyframe evaluation (linear, auto-Bezier spatial, all muted), serialization and value replacement, engine command + undo, Lottie export.

crates/engine/src/tests/menu_cmds.rs

@@ -1483,6 +1483,32 @@ fn key_selection_toggles_and_skips_locked_layers_and_hold_restores_bezier() {
     assert!(s.state.selected_keys.is_empty());
 }
 
+#[test]
+fn muting_keys_skips_them_in_the_timeline_value_and_undoes() {
+    let mut s = comp();
+    let a = solid(&mut s, "#ff0000");
+    for (t, v) in [(0.0, 0.0), (1.0, 100.0), (2.0, 300.0)] {
+        s.execute("prop.addKey", json!({"layer": a, "path": "transform/opacity", "time": t, "value": v})).unwrap();
+    }
+    let value_at = |s: &Session, t: f64| layer(s, a).props.prop("transform/opacity").unwrap().value_at(Tick::from_seconds_f64(t)).as_f64();
+    let select = |s: &mut Session| s.execute("keys.select", json!({"keys": [{"layer": a, "path": "transform/opacity", "time": 1.0}]})).unwrap();
+    select(&mut s);
+    let before = value_at(&s, 0.5);
+    s.execute("keys.setMuted", json!({"muted": true})).unwrap();
+    assert!(layer(&s, a).props.prop("transform/opacity").unwrap().keys[1].muted);
+    // The muted key no longer bends the 0 → 300 segment.
+    assert!((value_at(&s, 0.5) - 75.0).abs() < 1e-6);
+    assert!((value_at(&s, 1.5) - 225.0).abs() < 1e-6);
+    s.undo();
+    assert!(!layer(&s, a).props.prop("transform/opacity").unwrap().keys[1].muted);
+    assert!((value_at(&s, 0.5) - before).abs() < 1e-6);
+    // The same command unmutes.
+    select(&mut s);
+    s.execute("keys.setMuted", json!({"muted": true})).unwrap();
+    s.execute("keys.setMuted", json!({"muted": false})).unwrap();
+    assert!(!layer(&s, a).props.prop("transform/opacity").unwrap().keys[1].muted);
+}
+
 #[test]
 fn transform_drags_apply_their_whole_offset_to_the_keys_they_started_with() {
     let mut s = comp();