region: add a WedgeZ primitive

later i use this to Intersect a torus,
thereby creating a torus which only sweeps out one particular arc angle.

crates/coremem/src/geom/region/primitives.rs

@@ -36,6 +36,31 @@ impl Display for CylinderZ {
     }
 }
 
+/// describes all 3d space which falls within a given angular space, relative to the Z axis.
+#[derive(Copy, Clone, Default, Serialize, Deserialize)]
+pub struct WedgeZ {
+    arg_min: f32,
+    arg_max: f32,
+}
+
+impl WedgeZ {
+    pub fn new(arg_min: f32, arg_max: f32) -> Self {
+        Self { arg_min, arg_max }
+    }
+}
+
+impl Region for WedgeZ {
+    fn contains(&self, p: Meters) -> bool {
+        let arg = p.xy().arg();
+        // arg is [-pi, pi).
+        // if the user supplied some desired range where arg_max > pi, then we need to rotate
+        // one revolution "into" that range.
+        let arg_next = arg + f32::two_pi();
+        (arg >= self.arg_min && arg <= self.arg_max) ||
+        (arg_next >= self.arg_min && arg_next <= self.arg_max)
+    }
+}
+
 #[derive(Copy, Clone, Debug, Default, Serialize, Deserialize)]
 pub struct Torus {
     center: Meters,