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>

Sources/MotionLab/Renderer.swift

@@ -34,6 +34,7 @@ final class Renderer: NSObject, MTKViewDelegate {
     private var result: MTLTexture?  // the finished frame
     private var lastSerial = -1
     private let startTime = CACurrentMediaTime()
+    private var lastTime: Float?
     private var frameCount = 0
     private var pixels: [UInt8] = []
 
@@ -224,6 +225,8 @@ final class Renderer: NSObject, MTKViewDelegate {
         analysis.update(commands, camera: frames.current, particles: layers.contains { $0.enabled && $0.usesParticles })
 
         let time = Float(CACurrentMediaTime() - startTime)
+        let dt = min(max(time - (lastTime ?? time - 1.0 / 30.0), 0.001), 0.25)
+        lastTime = time
         var image = frames.current
         for layer in layers where layer.enabled {
             guard let pipeline = layer.pipeline else { continue }
@@ -231,8 +234,8 @@ final class Renderer: NSObject, MTKViewDelegate {
                 layer.targets = [makeTarget(device, width: width, height: height), makeTarget(device, width: width, height: height)]
                 for target in layer.targets { commands.pass(into: target, clear: true) }
             }
-            // must match `struct Uniforms` in the shader: time, padding, resolution.xy, params[16]
-            var uniforms: [Float] = [time, 0, Float(width), Float(height)]
+            // must match `struct Uniforms` in the shader: time, dt, resolution.xy, params[16]
+            var uniforms: [Float] = [time, dt, Float(width), Float(height)]
             uniforms += layer.params.map(\.value) + [Float](repeating: 0, count: maxParams - layer.params.count)
             let textures = [image, frames.current, frames.last, layer.targets[1], analysis.flow, analysis.particles]
             commands.pass(into: layer.targets[0]) { $0.fullscreen(pipeline, textures: textures, uniforms: uniforms) }

Sources/MotionLab/Shaders.swift

@@ -74,6 +74,7 @@ func effectShader(body: String, params: [EffectParam]) -> String {
     return shaderPrelude + """
     struct Uniforms {
         float time;
+        float dt;
         float2 resolution;
         float params[\(maxParams)];
     };
@@ -89,6 +90,7 @@ func effectShader(body: String, params: [EffectParam]) -> String {
         texture2d<float> particles;  // .r = brightness of particles that drift along with the motion
         sampler smp;
         float time;                  // seconds since start
+        float dt;                    // seconds since the previous frame (about 1/30)
         float2 resolution;           // output size in pixels
         Params p;                    // this effect's sliders
     };
@@ -106,7 +108,7 @@ func effectShader(body: String, params: [EffectParam]) -> String {
                                    constant Uniforms &u [[buffer(0)]]) {
         constexpr sampler smp(filter::linear, address::clamp_to_edge);
         Params p = { \(values) };
-        Inputs in = { image, camera, last, prev, flow, particles, smp, u.time, u.resolution, p };
+        Inputs in = { image, camera, last, prev, flow, particles, smp, u.time, u.dt, u.resolution, p };
         return effect(v.uv, in);
     }
     """

Sources/MotionLab/main.swift

@@ -51,6 +51,10 @@ if CommandLine.arguments.contains("--check") {
             _ = try makePipeline(device, source: effectShader(body: source, params: info.params),
                                  fragment: "effectFragment", format: .bgra8Unorm)
             print("ok    \(name)  sliders: \(info.params.isEmpty ? "none" : info.params.map(\.name).joined(separator: ", "))")
+            let sliderLines = source.split(separator: "\n").filter { $0.hasPrefix("// slider") }.count
+            if sliderLines != info.params.count {
+                print("      warning: \(sliderLines - info.params.count) slider line(s) ignored (malformed, or over the limit of \(maxParams))")
+            }
         } catch {
             failures += 1
             print("FAIL  \(name)\n\(error.localizedDescription)")