stim: convert CurlStimulus to a CurlVectorField and use ModulatedVectorField

this opens the door to caching the vector field stuff.

crates/applications/sr_latch/src/main.rs

@@ -7,7 +7,7 @@ use coremem::{Driver, mat, meas};
 use coremem::geom::{Coord as _, Meters, Torus};
 use coremem::sim::spirv::{SpirvSim, WgpuBackend};
 use coremem::sim::units::Seconds;
-use coremem::stim::{CurlStimulus, Sinusoid, StimExt as _};
+use coremem::stim::{CurlVectorField, ModulatedVectorField, Sinusoid, StimExt as _};
 
 
 fn main() {
@@ -81,9 +81,9 @@ fn main() {
         let wave = Sinusoid::from_wavelength(amp, duration * 2.0)
             .half_cycle()
             .shifted(start);
-        driver.add_stimulus(CurlStimulus::new(
-            region.clone(),
-            wave.clone(),
+        driver.add_stimulus(ModulatedVectorField::new(
+            CurlVectorField::new(region.clone()),
+            wave,
         ));
     };