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fix: address Copilot review findings

- hitTest: return hitNone for diff pane top border (y==0) instead of
  hitHeader, matching the tree pane symmetry and the hitNone contract
  for borders/gaps.
- wheelStepFor: use treePageSize/2 for shift+wheel over the tree/TOC
  pane so mouse half-page matches the keyboard half-page for that pane,
  instead of viewport.Height/2 (which is the diff half-page).
- handleMouse: guard wheel events on MouseActionPress for symmetry with
  the left-click branch, preventing double-handling if a terminal emits
  non-press wheel events.