Add in.dt, slider warnings in --check, and two agent-written effects

Spec fixes come from a test where pi + deepseek-v4.1-flash wrote effects
from EFFECTS.md alone: glitch-blocks and ghost-stamps both compiled on the
first try. The agent's feedback led to in.dt (frame time), documented
limits of in.prev as memory, and a --check warning for ignored slider lines.

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

effects/ghost-stamps.metal

@@ -0,0 +1,34 @@
+// Every `stamp` seconds the moving parts of the picture are frozen in place as a translucent,
+// tinted ghost; each ghost fades over `life` seconds, so moving things leave a row of copies.
+// order 70
+// slider stamp 0.2 3.0 0.5
+// slider life 0.2 6.0 2.0
+// slider hue 0.0 1.0 0.55
+// 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) {
+    // The frozen ghosts are one number per pixel (a tinted brightness), so they live in the
+    // alpha channel of this effect's own output. The colour is only added when displaying, so
+    // `in.prev.rgb` never feeds the live picture back into itself.
+    float ghosts = in.prev.sample(in.smp, uv).a;
+
+    float frame = in.dt;
+    ghosts *= exp(-frame / max(in.p.life, 0.05));
+
+    float3 live = in.image.sample(in.smp, uv).rgb;
+    float diff = length(in.camera.sample(in.smp, uv).rgb - in.last.sample(in.smp, uv).rgb);
+    float moving = smoothstep(0.03, 0.30, diff);
+    float fresh = dot(live, float3(0.299, 0.587, 0.114)) * moving;
+
+    // A stamp is taken just after each `stamp`-second slot of in.time begins. The window is two
+    // frames wide, so a stamp is not missed when the frame rate falls below the nominal one.
+    float slot = max(in.p.stamp, 0.05);
+    float begins = 1.0 - step(2.0 * frame / slot, fract(in.time / slot));
+    ghosts = max(ghosts, fresh * begins);
+
+    float3 tint = hueColor(in.p.hue);
+    return float4(live + tint * ghosts * 0.85, ghosts);
+}