Prove pixels unchanged

Choose the gate

• Byte-exact: the change keeps the same decoded data and the same shading math, so every pixel must match.
• Side-by-side review: the change alters sampled values, random order, precision, or where math runs. Report it as "not byte-gateable" and show old and new at each pose; do not invent thresholds.

Make frames deterministic

• Load every asset and match camera, canvas size, lighting, and animation time between builds. Seed incidental randomness.
• Drive animated shaders from one app-owned time value, not Date.now() or performance.now().
• app.timeScale = 0 makes dt zero, but update still runs; anything counting frames or reading wall-clock time still advances.
• Set app.autoRender = false, then app.renderNextFrame = true before each frame to capture.

Capture and compare

• WebGL: await (device as WebglGraphicsDevice).readPixelsAsync(x, y, w, h, pixels). WebGPU: render into a RenderTarget and read its colorBuffer with Texture#read(). Avoid colour conversion and lossy encoding.
• Capture the unmodified build twice first. The captures must match exactly, or the harness itself is noisy.
• Cover every touched surface in at least two poses, plus a mid-animation phase where relevant.
• Report the count, for example "0 of 65536 pixels differ", or the count and where the differences are. "Looks the same" is not a result.

Keep harness code out of the shipped bundle; attach it through inspect-runtime.