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>

effects/halftone.glsl

@@ -0,0 +1,17 @@
+// Halftone dots sized by brightness; dots turn hot orange wherever something moves.
+uniform float cell;  // 4.0 .. 40.0 = 10.0
+uniform float heat;  // 0.0 .. 6.0 = 3.0
+
+void main() {
+    vec2 grid = resolution / cell;
+    vec2 centre = (floor(uv * grid) + 0.5) / grid;
+    vec3 c = texture(camera, centre).rgb;
+    float luma = dot(c, vec3(0.299, 0.587, 0.114));
+
+    float radius = 0.5 * sqrt(luma);
+    float dist = length(fract(uv * grid) - 0.5);
+    float mask = 1.0 - smoothstep(radius - 0.05, radius + 0.05, dist);
+
+    float moving = clamp(texture(motion, centre).r * heat, 0.0, 1.0);
+    color = vec4(mix(c, vec3(1.0, 0.35, 0.1), moving) * mask, 1.0);
+}

effects/liquid.glsl

@@ -0,0 +1,13 @@
+// Liquid trails: the previous output is dragged along the motion, swirled a little,
+// and slowly fades back to the live camera.
+uniform float persistence;  // 0.0 .. 0.99 = 0.92
+uniform float drag;         // 0.0 .. 6.0 = 2.5
+uniform float swirl;        // 0.0 .. 0.01 = 0.002
+
+void main() {
+    vec2 centred = uv - 0.5;
+    vec2 from = uv - texture(flow, uv).xy * drag + vec2(-centred.y, centred.x) * swirl;
+    vec3 trail = texture(prev, from).rgb;
+    vec3 live = texture(camera, uv).rgb;
+    color = vec4(mix(live, trail, persistence), 1.0);
+}

effects/wave-shift.glsl

@@ -0,0 +1,12 @@
+// RGB split that ripples down the image and gets pushed around by your movement.
+uniform float amount;  // 0.0 .. 0.05 = 0.012
+uniform float waves;   // 1.0 .. 80.0 = 24.0
+uniform float react;   // 0.0 .. 3.0 = 1.0
+
+void main() {
+    vec2 d = vec2(amount * sin(uv.y * waves + time * 3.0), 0.0);
+    d += texture(flow, uv).xy * react;
+    color = vec4(texture(camera, uv + d).r,
+                 texture(camera, uv).g,
+                 texture(camera, uv - d).b, 1.0);
+}