fix(shader): apply node opacity to RadialProgress ring/track

[chiefcll]

Follow-up to #112 (the gradient transparency fixes). While auditing the other gradient/effect shaders for the same alpha-compositing issue, RadialProgress had it too.

Problem

The ring/track colors come from u_colors / u_trackColor, which do not carry worldAlpha. The shader composited them over base but never applied u_alpha, so a fading RadialProgress node kept its ring/track fully opaque — node opacity was ignored for the effect.

Fix

Scale the premultiplied layer (ring + track) by u_alpha before the "over" composite. base already includes worldAlpha via v_color, so it's left untouched to avoid double-applying. Scaling a premultiplied color by a scalar is a valid group-opacity operation.

Notes

• Identity at alpha == 1, so all existing snapshots are unchanged — verified with a visual-regression compare run (shader-radial-progress PASS, 0 diffs).
• getGradientColor already returns on all paths (unlike the LinearGradient case in fix(shader): correct linear & radial gradient transparency #112), so no missing-return fix needed here.
• Audit result for the rest: HolePunch declares u_alpha but doesn't apply it — that's correct, since it only masks existing v_color content (which already carries opacity); applying u_alpha there would double it. Rounded/Border/Shadow already apply u_alpha.

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

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