plot: Don't track hover through occluding overlays for tooltips (#2581)

## Description

Previously, the `IntoPlot` derive's mouse-move handler used a plain
`bounds.contains(&e.position)` test to track the cursor for tooltips.
This meant the tooltip (and its crosshair) kept following the mouse even
when an occluding overlay — such as an open popup menu or modal — was
above the plot.

This PR makes hover detection occlusion-aware:

- `prepaint` now inserts a `Hitbox` (with `HitboxBehavior::Normal`) for
the plot bounds and carries it to `paint` via `PrepaintState`.
- The mouse-move handler uses `hitbox.is_hovered(window)` instead of a
raw bounds test, so the tooltip clears when another hitbox (popup,
modal, etc.) is over the cursor.

Only affects the generated code in `crates/macros`; no public API
changes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>