Add getBarRegion: where a bar fits beside the status icons

[JanMichaelPeter]

What

WindowPlacementDetector.getBarRegion(WindowEdge edge, double extent) returns where the system places a bar of that thickness along a window edge, as a Rect in Flutter's coordinates, clear of the status icons, the rounded corners and the home indicator. It wraps UIKit's UIView.LayoutRegion.bar(onEdge:extent:) (iOS 27.1).

Why

On the iPhone Duo the status icons sit in a column at the side, or in a band at the top in unfolded portrait. The safe area gives up that whole column or band; there is no public API for the icons' frames, and statusBarFrame is a 2 pt strip there. The bar region is the only API that knows where the icons end: a bar on their edge starts below them, a top bar ends before them.

Measured on the iPhone Duo simulator (iOS 27.1), 64 pt bars:

Posture	Safe insets	Bar region next to the icons
Folded	right 84, bottom 34	right bar from y 170
Unfolded landscape	right 84, bottom 34	right bar from y 120
Split screen, app on the right	right 84, bottom 34	right bar from y 120
Split screen, app on the left	bottom 34	right bar from y 16 (no icons)
Unfolded portrait	top 82, bottom 34	top bar x 16–535

How

• New Pigeon host method getBarRegion(BarEdge, double) -> BarRegion? (regenerated with Pigeon 27.3.0, as before).
• iOS: looks up the Flutter view in the active window and reads the layout guide's frame. Null before iOS 27.1.
• Android: returns null (no equivalent).
• The platform interface defaults to null, so implementations that extend it keep working; mocks that implements it need the method.
• Public WindowEdge enum, README section, CHANGELOG, version 0.0.3.

Testing

• Unit tests for the Pigeon mapping and the detector delegation; analyze, tests and the format check pass on Flutter 3.35.6.
• On the simulator, the plugin returned exactly the rectangles UIKit reports directly, on all four edges.

duo_navigation uses this to keep its side column and title bar clear of the status icons (separate PR, needs this released as 0.0.3).