* fix(ui): reserve chart readout height to prevent layout shift

Reserve initial readout row height in interactive charts using FontMetrics
so that hovering a point for the first time does not cause a vertical layout shift.

Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>

* fix(ui): reserve chart readout height to prevent layout shift

Reserve initial readout row height in interactive charts using FontMetrics
so that hovering a point for the first time does not cause a vertical layout shift.

Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>

* test(ui): verify chart readout geometry with real controls

Cover first pointer and keyboard inspection, exit, and cleared points. Keep unrelated plain-text control tests unchanged. Document the regression coverage; the original reserved-height change has no observed geometry difference on the tested Qt 6.8.2 and 6.11.1 environments.

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Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>