Add Shape controls and vector geometry

[ProGraMajster]

Summary

Adds a complete, platform-neutral Shape and vector-geometry subsystem to ModernFormsNext. The implementation follows the existing Skia rendering, Brush, layout, input, accessibility, and Designer pipelines without introducing platform types into shared projects.

Closes #11.

Implementation

• Adds the Shape base control and Ellipse, Circle, Line, Polygon, Polyline, and Path controls.
• Adds a platform-neutral Geometry model with ellipse, rectangle, line, and path geometries.
• Adds path figures and line, quadratic Bézier, and cubic Bézier segments.
• Supports fill rules, point collections, geometry transforms, and mutable collection notifications.
• Preserves Circle semantics while keeping stroke rendering safe at small sizes.

Designer integration

• Registers Shape controls with the Toolbox and Property Grid.
• Adds structured editors for PointCollection and PathGeometry.
• Adds Designer defaults, serialization, code generation, reverse parsing, and round-trip coverage.
• Corrects ColorDialog client sizing used by the Designer brush workflow.

Rendering and hit testing

• Reuses the shared Brush and Skia pipelines for solid, linear, radial, and sweep fills and strokes.
• Supports stroke thickness, caps, joins, clipping, presentation transforms, and antialiased vector rendering.
• Adds geometry-aware pointer hit testing for transparent fill regions and thin strokes.
• Invalidates cached Skia paths and paints when mutable Geometry or Brush content changes.
• Keeps the implementation shared by Windows and Android backends.

Serialization and code generation

• Extends .mfdesign persistence for Shape controls, points, figures, segments, transforms, fill rules, and brush values.
• Generates code for structured vector geometry.
• Extends reverse parsing so generated Shape code can be reopened by the Designer.
• Adds compatibility aliases where the public ModernFormsNext.Path name would otherwise conflict with System.IO.Path.

Testing

• dotnet restore .\ModernFormsNext.slnx
• Full Debug solution build, including the Android backend and VSIX validation.
• Full Release solution build, including the Android backend and VSIX validation.
• Explicit Debug and Release VSIX builds.
• Full test suite: 1062/1062 passed, 0 failed, 0 skipped.
• git diff --check passed.
• Validation was run in a clean isolated copy to avoid live Designer host locks and nested worktree contamination.

Manual validation

Manually verified on Windows:

• ControlGallery Shapes page, including resize and move behavior.
• Smooth transformed Path rendering after Geometry.Transform.
• PointCollection and PathGeometry structured editors.
• Toolbox and Property Grid integration.
• All Shape controls, including Circle sizing semantics.
• Fill, stroke, gradients, caps, joins, clipping, transforms, and geometry-aware hit testing.

Known limitations

• Android build and automated tests are validated, but no manual Android emulator or physical-device run is claimed.
• PathGeometry uses a structured Designer editor; this change does not add a graphical Bézier canvas editor.