Don't repaint comments when no highlight moved

[Fyko]

Symptom

Open a document whose own content mutates on a timer (e.g. a live clock doing el.textContent = ... every second) and the comments/annotations sidebar flashes on every tick — even though nothing about the comments changed.

Root cause

The runtime injected into the document watches document.body with a MutationObserver (collab-client.ts) so highlights stay anchored as the document reflows. Any mutation — including a one-character text swap from a clock — schedules a re-measure and posts a fresh highlights:positions report to the shell.

In the shell, the non-animated branch of the highlights:positions handler (shell-client.ts) sets shouldRender = true unconditionally:

} else {
  highlightPixelPositions = nextPixelPositions;
  shouldRender = true;
}

So every report triggers renderComments(), repainting the sidebar even when every anchor is in the exact same place. A ticking document = a repaint per tick = the flash.

Fix

Diff the incoming positions against the current ones and only repaint when something actually moved:

} else if (!pixelPositionsEqual(highlightPixelPositions, nextPixelPositions)) {
  highlightPixelPositions = nextPixelPositions;
  shouldRender = true;
}

Real reflows (a highlight added/removed, content shifting an anchor) still report different positions and repaint as before; only the no-op reports are dropped. This also quiets unrelated spurious reports (resize jitter, etc.), not just clocks.

The document runtime still re-measures on mutation (cheap), it just no longer forces a paint when the result is identical.

Tests

pixelPositionsEqual extracted as a pure helper with unit coverage (identical / moved / added / removed). 50 passed, typecheck clean.