Compositions: reject zero and negative frame rates instead of panicking

comp.new / comp.settings passed `frameRate` straight to
FrameRate::from_f64, so 0 (or anything below 0.0005) became a 0/1 rate
and the next frame-duration computation divided by zero, killing the
process (and an MCP session with its unsaved project); a negative rate
was accepted as-is. Scripts hit it via addComp(..., 0) or
comp.frameRate = 0. Return a parameter error like renderQueue.add's
frameRate check does.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

crates/engine/src/commands/comp.rs

@@ -12,6 +12,14 @@ pub(crate) fn rate_p(p: &Value) -> Option<FrameRate> {
     f_p(p, "frameRate").or(f_p(p, "fps")).map(FrameRate::from_f64)
 }
 
+/// A zero, negative or tiny rate would make a zero-length frame (and divide by zero later).
+fn check_rate(p: &Value, cmd: &str) -> Result<()> {
+    match f_p(p, "frameRate").or(f_p(p, "fps")) {
+        Some(f) if !(0.001..=1000.0).contains(&f) => Err(bad(cmd, "frameRate: fps from 0.001 to 1000")),
+        _ => Ok(()),
+    }
+}
+
 fn apply_settings(c: &mut Comp, p: &Value) {
     let (ow, oh) = (c.width, c.height);
     if let Some(w) = p.get("width").and_then(Value::as_u64) {
@@ -93,6 +101,7 @@ fn apply_settings(c: &mut Comp, p: &Value) {
 }
 
 fn new_comp(s: &mut Session, p: &Value) -> Result<Value> {
+    check_rate(p, "comp.new")?;
     let name = str_p(p, "name").unwrap_or("Comp 1").to_string();
     let rate = rate_p(p).unwrap_or(FrameRate::FPS_29_97);
     let mut c = Comp::new(1920, 1080, rate, rate.snap_nearest(Tick::from_seconds_f64(10.0)));
@@ -119,6 +128,7 @@ fn new_comp(s: &mut Session, p: &Value) -> Result<Value> {
 }
 
 fn settings(s: &mut Session, p: &Value) -> Result<Value> {
+    check_rate(p, "comp.settings")?;
     let cid = comp_id(s, p)?;
     s.edit("Composition Settings", None, |proj, _| {
         if let Some(n) = str_p(p, "name")

crates/engine/src/tests.rs

@@ -301,6 +301,17 @@ fn comp_settings_anchor_start_timecode_renderer() {
     assert_eq!(c.renderer, crate::project::Renderer::Advanced3D);
 }
 
+#[test]
+fn comp_rejects_zero_and_negative_frame_rates() {
+    let mut s = Session::default();
+    for fps in [0.0, -30.0, 0.0001] {
+        assert!(s.execute("comp.new", json!({"frameRate": fps})).is_err(), "{fps}");
+    }
+    s.execute("comp.new", json!({"frameRate": 30, "duration": 1})).unwrap();
+    assert!(s.execute("comp.settings", json!({"frameRate": 0})).is_err());
+    assert_eq!(s.active_comp().unwrap().frame_rate, crate::time::FrameRate::FPS_30);
+}
+
 #[test]
 fn render_queue_add_accepts_documented_settings_when_checked() {
     let mut s = demo();