M1: refuse negative vector sizes

A negative width or height was clamped to zero by the scale helper, producing a layer smaller than asked for; vector.create now says so. Regression test covers width -10 with height 8.

crates/engine/src/cmd/create.rs

@@ -170,8 +170,13 @@ pub fn parse_path_data(d: &str) -> Option<figcraft_geom::BezPath> {
 /// independently (as resizing a vector does); with one, the other keeps its proportion. A size
 /// asked for on an axis the geometry has none of is an error, not silently ignored.
 fn vector_scale(width: Option<f64>, height: Option<f64>, bw: f64, bh: f64, what: &str) -> std::result::Result<(f64, f64), String> {
+    for (axis, asked) in [("width", width), ("height", height)] {
+        if asked.is_some_and(|a| a < 0.0) {
+            return Err(format!("`{axis}` can't be negative"));
+        }
+    }
     let fit = |asked: Option<f64>, have: f64, axis: &str| match asked {
-        Some(a) if have > 1e-9 => Ok(a.max(0.0) / have),
+        Some(a) if have > 1e-9 => Ok(a / have),
         Some(_) => Err(format!("`{axis}` can't size {what} with no {} extent", if axis == "width" { "horizontal" } else { "vertical" })),
         None => Ok(1.0),
     };

crates/engine/src/vector_behaviour_tests.rs

@@ -468,7 +468,9 @@ fn vector_paths_take_the_requested_size_and_refuse_empty_geometry() {
     assert!(e.contains("horizontal extent"), "{e}");
     let e = s.execute("vector.create", &json!({"path": "M0 0 L24 0", "height": 16})).unwrap_err().to_string();
     assert!(e.contains("vertical extent"), "{e}");
-    // A size that would collapse both axes is refused too.
+    // A size that would collapse both axes is refused too, and so is a negative one.
     let e = s.execute("vector.create", &json!({"path": path, "width": 0})).unwrap_err().to_string();
     assert!(e.contains("0×0"), "{e}");
+    let e = s.execute("vector.create", &json!({"path": path, "width": -10, "height": 8})).unwrap_err().to_string();
+    assert!(e.contains("can't be negative"), "{e}");
 }

docs/mcp.md

@@ -429,8 +429,8 @@ on; `key` resolves keys the way the canvas does. Commands behind keys:
 `vector.create {path | points}` makes the layer's box from the geometry; `width`/`height` scale
 the geometry to the size asked for (with both, each side independently; with one, the other keeps
 its proportion). A path or point set that draws nothing is refused instead of making an invisible
-0×0 layer, as is a size on an axis the geometry has none of and a size that would collapse both
-axes.
+0×0 layer, as is a size on an axis the geometry has none of, a negative size, and a size that
+would collapse both axes.
 
 Vector edit mode is driven with `run_command` (or `pointer`/`key` for gestures):
 `vector.edit.begin {id? | ids?}` (default the selection; converts shapes to vectors; several layers