fix: hide zero-opacity elements at runtime to reduce GPU layers

[bertpl]

Summary

• Renderer emits data-opacity-keyframes attribute on elements with animated opacity.
• New ElementVisibility class evaluates the piecewise-linear opacity function on each scroll tick and toggles visibility: hidden when opacity drops below threshold (0.001).
• Only writes to DOM at threshold crossings, not every tick.
• Particularly impactful for scrollimation slides with many stacked images where only 2-3 are visible at any scroll position.

Test plan

• 646 Python + 103 JS tests pass (including new evaluatePiecewiseLinear tests)
• Built worked example - elements at opacity 0 have visibility:hidden in DevTools
• Scroll through scrollimation slides - transitions remain smooth

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