Tests: 22 covering grid/compile/eval/engine (was 0; orca-rs has 11)

grid: ruler_snap, resize clamps, bounds, fixed-stride preservation.
dsp:  op mapping, input/param/CC wiring, env detection, sine bounds,
      exact saw/mix samples, output summing, note gating, envelope
      rise+release, cutoff curve.
engine: A4->440, oldest-voice steal, note reaping, CC registers, limiter.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GLUwWE2KmFPzhKaf67tWbx

src/engine.rs

@@ -91,3 +91,75 @@ impl Engine {
         (mix * 0.3).tanh()
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::dsp::compile;
+    use crate::grid::Grid;
+
+    #[cfg(test)]
+    impl Engine {
+        fn first_freq(&self) -> Option<f32> {
+            self.voices.iter().find(|v| v.active).map(|v| v.freq)
+        }
+        fn reg(&self, i: usize) -> f32 {
+            self.regs[i]
+        }
+    }
+
+    fn grid_from(src: &str) -> Grid {
+        let mut g = Grid::new();
+        for (r, line) in src.lines().enumerate() {
+            for (c, ch) in line.chars().enumerate() {
+                g.set(c, r, ch);
+            }
+        }
+        g
+    }
+
+    #[test]
+    fn noteon_maps_a4_to_440() {
+        let mut e = Engine::new(4, 48000.0);
+        e.apply(Event::NoteOn { note: 69, vel: 100 });
+        assert!((e.first_freq().unwrap() - 440.0).abs() < 0.01);
+        assert_eq!(e.active_voices(), 1);
+    }
+
+    #[test]
+    fn noteoff_reaps_gated_voice() {
+        let p = compile(&grid_from("A\n!")); // no envelope -> gate reaps immediately
+        let mut e = Engine::new(4, 48000.0);
+        e.apply(Event::NoteOn { note: 60, vel: 100 });
+        e.render(&p);
+        assert_eq!(e.active_voices(), 1);
+        e.apply(Event::NoteOff { note: 60 });
+        e.render(&p);
+        assert_eq!(e.active_voices(), 0);
+    }
+
+    #[test]
+    fn monophonic_steals_the_oldest_voice() {
+        let mut e = Engine::new(1, 48000.0);
+        e.apply(Event::NoteOn { note: 60, vel: 100 });
+        e.apply(Event::NoteOn { note: 72, vel: 100 });
+        assert_eq!(e.active_voices(), 1);
+        assert!((e.first_freq().unwrap() - 440.0 * 2f32.powf((72.0 - 69.0) / 12.0)).abs() < 0.01);
+    }
+
+    #[test]
+    fn cc_writes_register() {
+        let mut e = Engine::new(2, 48000.0);
+        e.apply(Event::Cc { idx: 0, val: 127 });
+        assert!((e.reg(0) - 1.0).abs() < 1e-6);
+    }
+
+    #[test]
+    fn master_output_is_limited() {
+        // many stacked saws would exceed 1.0 without the limiter
+        let p = compile(&grid_from("A.A.A.A\n!.!.!.!"));
+        let mut e = Engine::new(4, 48000.0);
+        e.apply(Event::NoteOn { note: 60, vel: 127 });
+        assert!(e.render(&p).abs() <= 1.0);
+    }
+}