perf(shaders): move RoundedWithBorder border math from varyings to CPU uniforms

[chiefcll]

What

Rewrites the WebGL RoundedWithBorder shader so all border geometry is computed once on the CPU and uploaded as uniforms, instead of being recomputed in the vertex shader and shipped to the fragment stage through varyings.

Why

All nine border varyings (v_innerSize, v_outerSize, v_outerBorderUv, v_innerBorderUv, v_innerBorderRadius, v_outerBorderRadius, v_halfDimensions — 18 interpolated floats) were derived purely from uniforms, so they were constant across the quad yet paid per-fragment interpolation cost on every pixel. On the embedded GPUs this engine targets, interpolator pressure is real fragment-pipeline cost.

This matters most for the common TV-app pattern of border: {w: 0} on every card until focus: with a zero-width border, the fragment shader now carries the same 3 varyings as the plain Rounded shader, making the per-pixel cost of an unfocused card identical to Rounded. The full border path only runs its math when border props or node dimensions actually change (cached by the existing shader value-key cache), i.e. once per focus change rather than per frame.

How

• calcBorderShaderValues() mirrors the previous vertex-shader expressions exactly (adjusted widths, gap handling, inner/outer sizes, UV offsets, inner/outer radii, edge offset) and writes into a reused scratch buffer.
• borderZero is precomputed and uploaded as a uniform instead of re-derived per fragment via dot/step.
• The vertex shader's border branch is replaced with arithmetic on that flag; the quad-expansion logic (edge + edge offset) is preserved.
• With a zero border, update() skips computing/uploading the border uniforms entirely (both shader stages early-out before reading them).

GLSL output is unchanged by construction. No prop/API changes; the Canvas2D backend is untouched.

Testing

• pnpm build passes.
• New co-located unit tests (RoundedWithBorder.test.ts) verify the CPU math against hand-computed cases: uniform/asymmetric borders, gap, align inside/center/outside, sub-pixel width collapse, and negative inner-radius clamping.
• Locally loaded ?test=shader-border in the examples app: shaders compile and render with no GL errors.
• Visual certification deferred to CI's visual regression run (snapshots should be pixel-identical).

🤖 Generated with Claude Code