fix(shaders): don't re-convert raw canvas shaders

[chiefcll]

Problem

Raw Canvas shaders still go through convertToShader when set on each node, even though a raw shader already carries everything it needs to render. WebGL handles this correctly; Canvas does not.

The guard in ElementNode._render was:

if (SHADERS_ENABLED && props.shader && !props.shader.program) {
  props.shader = Config.convertToShader(node, props.shader);
}

A CanvasShaderNode exposes render, not program, so it falls through the !props.shader.program check and gets wrongly re-run through convertToShader. WebGL only works because WebGlShaderNode happens to have a program field.

Solution

This is an alternative to #34. Instead of widening the check to 'program' in shader || 'render' in shader (which enumerates backend-specific fields and needs a new clause per backend), use the single field that every built shader shares and a raw StyleEffects props object never has: shaderType.

• WebGlShaderNode and CanvasShaderNode both extend CoreShaderNode, which declares readonly shaderType.
• The DOM-renderer test fake returns { shaderType, props, program: {} }.
• StyleEffects (border*/shadow/rounded/gradients/holePunch) has no shaderType.

So !('shaderType' in props.shader) cleanly means "this is a raw style-props object, build a shader from it" and covers WebGL, Canvas, and the DOM test path uniformly.

Behavior parity

props.shader	has shaderType?	result
WebGL node	yes	skip (same as before)
Canvas node	yes	skip (fixes the bug)
DOM test fake	yes	skip
StyleEffects props	no	convert

Verification

• npm run tsc — clean
• npm test — 134 passed
• npm run lint — 0 errors

🤖 Generated with Claude Code