Add FilledStrokedRectangle: flat fill+stroke renderable for FRB's Rectangle codegen

[vchelaru]

Summary

Adds RenderingLibrary.Math.Geometry.FilledStrokedRectangle — a flat IRenderableIpso that draws fill and stroke in one Render() call, gated independently (IsFilled for fill, StrokeWidth > 0 for stroke, neither hides the other). Built for FlatRedBall's Glue codegen, which emits one flat contained field per standard element and can't adopt RectangleRuntime's two-slot GraphicalUiElement composite.

Reuses the existing draw techniques directly — Sprite.Render for fill, LineRectangle.RenderLinePrimitive/UpdateLinePrimitive for stroke (both already static and generic over any IRenderableIpso) — instead of reimplementing either, so a fix to either draw path never needs hand-mirroring into a third place.

RectangleRuntime's XNALIKE two-slot composite is unchanged — it's still needed so the optional MonoGameGumShapes package can swap in richer per-slot renderables (gradient/dropshadow/rounded corners). This new type is scoped to plain square-corner fill + stroke only, matching the issue.

Wiring FRB's StandardsCodeGenerator to actually use this type is tracked separately in the FlatRedBall repo (#1907/#1967).

Test plan

• New unit tests in MonoGameGum.Tests/RenderingLibraries/Math/Geometry/FilledStrokedRectangleTests.cs (defaults, independent fill/stroke gating, parent/children, alpha)
• Full MonoGameGum.Tests suite green (2142/2142)
• KniGum builds clean, no new warnings
• FRB canary (GumCore.DesktopGlNet6.csproj, net6.0 + net8.0): 0 errors, matches baseline

Manual test: not needed — this is a new, standalone renderable type not wired into any existing runtime path (MonoGameGum's RectangleRuntime is untouched); correctness is covered by unit tests + the FRB canary compile.

Closes #4341