fix(shader): correct linear & radial gradient transparency

[chiefcll]

Fixes the linear gradient transparency issues from #111, with the WebGL portability/perf gaps addressed, and extends the fix to the radial gradient.

Problems

• WebGL linear gradient rendered solid colors as semi-transparent (gradient alpha was never composited into the output and node opacity was ignored).
• Canvas linear gradient forced transparent stops to solid. The "nearest opaque RGB" workaround read the high byte as alpha, but colors are RGBA (alpha is the low byte), so it overwrote the real alpha.

Changes

WebGL LinearGradient

• Restore the getGradientColor fallthrough return — a non-void GLSL function falling off the end is undefined behavior and a hard compile error on many embedded GLES2 drivers (lenient only on desktop Chrome).
• Composite gradient alpha into the output and apply node opacity via u_alpha.
• Keep the mediump-overflow fix, but precompute the gradient ramp as dist = dot(v_textureCoords, u_grad_a) + u_grad_b on the CPU instead of recomputing cos/sin/projection per fragment (the prior approach regressed fill rate against the TV perf model). Cache-safe — createValueKey already includes node-width/node-height.
• Drop the custom vertex shader; the Default.vertex fallback is geometrically identical.

Canvas LinearGradient

• Remove the broken byte-mislabeled workaround; map each stop straight through toColorString.

WebGL RadialGradient (follow-up)

• Apply the same alpha compositing and actually use the declared u_alpha (node opacity was previously ignored).

Tests

• Unit tests: canvas color mapping (per-stop alpha preserved) and the WebGL ramp math (endpoints map to 0/1). Both fail against the old code.
• New shader-linear-gradient-alpha visual test — gradients over a white backdrop covering solid-opaque, partial-alpha, fade-to-transparent, and 50% node opacity.
• Regenerated affected CI snapshots in the Docker runtime: shader-linear-gradient-1, shader-radial-gradient-1, and the new shader-linear-gradient-alpha-1. Visually verified.

pnpm build clean · 257/257 unit tests pass · prettier/eslint clean.

🤖 Generated with Claude Code