Go native-only: remove the Python POC, move the Swift app to the top level

Adds --check (compile every effect without camera or window), EFFECTS.md
(the contract for writing effects) and AGENTS.md. The POC stays available
at the 'poc' tag.

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

effects/neon-edges.metal

@@ -0,0 +1,38 @@
+// Glowing outlines with a slowly cycling colour.
+// order 20
+// slider detail 1.0 12.0 5.0
+// slider glow 0.0 6.0 2.5
+// slider source 0.0 1.0 0.35
+// slider hue 0.0 1.0 0.55
+// slider cycle 0.0 1.0 0.3
+// 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);
+}
+
+static float luma(Inputs in, float2 uv) {
+    return dot(in.image.sample(in.smp, uv).rgb, float3(0.299, 0.587, 0.114));
+}
+
+// how strongly brightness changes around this point
+static float edge(Inputs in, float2 uv) {
+    float2 t = 1.5 / in.resolution;
+    float dx = luma(in, uv + float2(t.x, 0.0)) - luma(in, uv - float2(t.x, 0.0));
+    float dy = luma(in, uv + float2(0.0, t.y)) - luma(in, uv - float2(0.0, t.y));
+    return length(float2(dx, dy));
+}
+
+float4 effect(float2 uv, Inputs in) {
+    float sharp = smoothstep(0.1, 0.5, edge(in, uv) * in.p.detail);
+    float halo = 0.0;  // edges found in a ring around this pixel = a cheap glow
+    for (int i = 0; i < 12; i++) {
+        float a = 6.2832 * float(i) / 12.0;
+        float2 dir = float2(cos(a), sin(a)) / in.resolution;
+        halo += edge(in, uv + dir * 4.0) + edge(in, uv + dir * 9.0);
+    }
+    halo = halo / 24.0 * in.p.detail * in.p.glow;
+
+    float3 tint = hueColor(in.p.hue + in.time * in.p.cycle * 0.2);
+    float3 base = in.image.sample(in.smp, uv).rgb * in.p.source;
+    return float4(base + tint * (sharp + halo), 1.0);
+}