Python/OpenCV proof of concept plus first native Swift port

Snapshot before the Python POC is removed. The POC (main.py, effects.py,
panel.py, shaders.py, effects/*.glsl, flake.nix) is the reference for the
native app under native/.

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

native/effects/motion-glow.metal

@@ -0,0 +1,20 @@
+// Whatever moves lights up and the glow lingers. The heat is carried from frame to frame
+// in the alpha channel of this effect's own previous output.
+// order 50
+// slider sensitivity 1.0 30.0 10.0
+// slider linger 0.5 0.99 0.93
+// slider dim 0.0 1.0 0.45
+// slider hue 0.0 1.0 0.07
+// hue 0..1 -> a fully saturated colour (0 red, 0.08 orange, 0.33 green, 0.66 blue)
+static float3 hueColor(float h) {
+    return clamp(abs(fract(h + float3(0.0, 2.0 / 3.0, 1.0 / 3.0)) * 6.0 - 3.0) - 1.0, 0.0, 1.0);
+}
+
+float4 effect(float2 uv, Inputs in) {
+    float moved = length(in.camera.sample(in.smp, uv).rgb - in.last.sample(in.smp, uv).rgb);
+    float fresh = smoothstep(0.3, 1.0, moved * in.p.sensitivity);
+    float heat = max(in.prev.sample(in.smp, uv).a * in.p.linger, fresh);
+
+    float3 glow = hueColor(in.p.hue);
+    return float4(in.image.sample(in.smp, uv).rgb * in.p.dim + glow * heat, heat);
+}