M0: truck-based kernel with local fillets and chamfers, parametric document timeline

The kernel moved from monstertruck to truck: truck's booleans handle blind
holes, pockets and bosses that monstertruck's did not (see the ADR). Fillets
and chamfers on straight convex edges between planar faces are our own exact
local B-rep operation. The document holds parameters with unit-aware
expressions and a timeline that re-evaluates incrementally.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/kernel/src/step.rs

@@ -1,18 +1,17 @@
-use monstertruck_io::step::save;
+use truck_stepio::out;
 
 use crate::body::Body;
 use crate::{KernelError, Result, guard};
 
-/// STEP (ISO 10303-21, AP203-style geometry) text for the given bodies.
+/// STEP (ISO 10303-21) text for the given bodies.
 pub fn step_export(bodies: &[&Body], system: &str) -> Result<String> {
     if bodies.is_empty() {
         return Err(KernelError::Invalid("nothing to export".into()));
     }
     guard("step export", || {
         let cs: Vec<_> = bodies.iter().map(|b| b.solid.compress()).collect();
-        let models = save::StepModels::from_iter(cs.iter());
-        let header = save::StepHeaderDescriptor { organization_system: system.to_owned(), ..Default::default() };
-        let out = save::CompleteStepDisplay::new(models, header).to_string();
-        Ok(out)
+        let models: out::StepModels<_, _, _> = cs.iter().collect();
+        let header = out::StepHeaderDescriptor { organization_system: system.to_owned(), ..Default::default() };
+        Ok(out::CompleteStepDisplay::new(models, header).to_string())
     })
 }