Hide the diff minimap when the diff barely scrolls

[mtsgrd]

The minimap ruler took a column on every diff, including ones a flick of the
wheel covers, where it says nothing the native scrollbar does not. Below three
windows of content it now stands down.

Expressed as a null geometry rather than a new flag: getMinimapGeometry
already returns null for "nothing worth mapping", so this path hides the ruler
and hands the scrollbar back with no new wiring — both the CSS and the geometry
doc already described this case, it was simply never enforced.

The threshold is measured in windows to match MAX_MAP_TRACKS next door, where
a track is a ruler-full. A window of zero is a pane that hasn't been laid out
yet rather than a short diff, so it is excluded from the test; the resize that
gives the pane a height brings the map back.

[Copilot]

Pull request overview

This PR updates the Lite diff minimap so it automatically hides (by returning null geometry) when the diff content is short enough that the native scrollbar provides sufficient navigation, restoring the scrollbar in those cases without adding new flags or wiring.

Changes:

• Introduces a “minimum mapped windows” threshold (MIN_MAPPED_WINDOWS = 3) to decide when the minimap is worth showing.
• Updates getMinimapGeometry to return null when the total diff content height is less than 3× the viewer height (excluding the pre-layout height === 0 case).

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