Preserve fractional scroll offset for pixel-smooth trackpad scrolling

[hmrserver]

This preserves the fractional viewport offset so trackpad scrolling can move at pixel granularity while rows remain the unit of layout and rendering.

Changes:

• Keep the row calculation intact, then store the leftover pixel offset as _pixelOffset = newRow * cellHeight - scrollTop.
• Apply that offset as a devicePixelRatio-rounded translateY() on the screen element.
• Add will-change: transform so the in-between motion stays on the compositor.
• Reset the offset when switching between the normal and alt buffers.
• Replace the WebGL addon's one-shot beginFrame() boolean with a monotonic clearModelGeneration counter. With a shared atlas, the boolean can be consumed by the first renderer that sees it; the counter lets each renderer observe and handle model clears independently.

Unaffected behavior:

• Selection still uses the same row model.
• Alt-buffer apps still reset to an aligned viewport.
• smoothScrollDuration is unchanged; this only affects the fractional offset inside each row step.

Demo recording:
BEFORE:

laggy.scroll.mp4

AFTER:

smooth.scroll.mp4