Add extensible Paint and gradient system

[ProGraMajster]

Problem

ModernFormsNext already exposed several brush and gradient types, but mutation, invalidation, resource sharing, coordinate semantics, and Skia shader creation were inconsistent. That made the existing surface difficult to reuse safely for dynamic themes and the planned Shape controls.

Current state

The framework used the Brush hierarchy directly and already supported solid, linear, radial, sweep, and glass rendering. Several public properties exposed SkiaSharp types, gradient stops were plain mutable data, shader construction was distributed through rendering extensions, and controls did not observe internal brush mutations.

Solution

• Keep the existing Brush hierarchy as the single canonical model instead of introducing a parallel Paint hierarchy.
• Make brushes, gradient stops, and the typed stop collection observable and shareable.
• Add opacity, platform-neutral transforms, relative gradient geometry, focal radial origins, pad/repeat/reflect spread modes, and NoBrush.
• Centralize bounds-aware Skia shader construction in SkiaBrushFactory.
• Observe mutable brushes from controls with weak, reference-counted subscriptions and render-only invalidation.
• Extend Designer serialization and editing so the new values survive .Designer.cs round trips.
• Add a focused ControlGallery page and user/architecture documentation.

Public API

The preferred platform-neutral API uses System.Drawing.Color, PointF, and System.Numerics.Matrix3x2. The extended surface includes Brush.Opacity, Brush.Transform, GradientSpreadMode, GradientStopCollection, relative geometry on linear/radial/sweep brushes, RadialGradientBrush.GradientOrigin, and NoBrush.

Existing SKColor and SKPoint members remain as compatibility views over the same backing values. They are documented as legacy integration surfaces; new application code does not need SkiaSharp types.

Compatibility

Existing BackColor, ForeColor, and legacy brush constructors continue to work. BackgroundBrush takes precedence when assigned; clearing it restores the color fallback, so there are not two independent rendering sources of truth.

The main compatibility risk is that GradientStops now returns GradientStopCollection rather than List<GradientStop>, and invalid offsets are rejected instead of flowing into rendering. Existing source that only uses normal collection operations remains straightforward to migrate, but already compiled consumers must rebuild.

Dynamic resources

Brush values participate in the existing application/window/control resource hierarchy. Tests cover hierarchy overrides, object replacement, in-place gradient mutation, removal and fallback, duplicate-subscription prevention, and release of controls without a durable brush subscription.

SkiaSharp integration

SkiaBrushFactory is the single conversion boundary. It maps spread modes to Skia tile modes, applies relative bounds and transforms once, and creates short-lived shaders owned by the drawing operation. Bounds-dependent native shaders are deliberately not cached, avoiding stale-size entries and unclear native lifetime.

Validation

• dotnet restore ModernFormsNext.slnx: passed.
• Debug and Release solution builds: passed.
• Full solution tests: 536/536 passed.
• Focused brush/gradient tests: 32/32 passed.
• Dynamic-resource and brush-resource tests: 20/20 passed.
• Designer serialization tests: 3/3 passed.
• Windows target build: passed.
• Android backend, cross-platform Android sample, and Android smoke-test builds: passed.
• Designer, VSIX Debug/Release, ControlGallery, and template/reference app builds: passed.
• Local package validation: eight 1.8.0 .nupkg files, seven expected .snupkg files, README and XML documentation present.

Known warnings are pre-existing dependency/security advisories in the VSIX graph, .NET Framework compatibility warnings for older Visual Studio SDK packages, nullable/generated interop warnings, obsolete Skia text/filter APIs, Windows platform annotations, and Android 16 KB page-size warnings for the bundled HarfBuzz native library. No warning was suppressed.

Windows impact

The primary Windows rendering and Designer paths use the shared brush factory. Designer code generation now preserves opacity, transform, spread mode, and radial focal origin while legacy brush expressions continue to deserialize.

Android impact

The Android backend consumes the same shared brush and Skia conversion layers. Its target builds and backend tests pass. Android remains experimental; GPU behavior, performance, lifecycle interactions, and rendering on physical devices still require manual validation.

ControlGallery

A dedicated Paint and Gradients panel demonstrates solid, linear, radial/focal, sweep, spread, transform, opacity, resource replacement, and in-place resource mutation. No screenshot is attached because an interactive visual session was not performed in this environment.

Limitations

• Gradient coordinates are relative-only in this stage.
• Advanced linear-light/color-space interpolation is not exposed yet.
• JSON theme serialization is designed and documented but intentionally not implemented without ThemeManager.
• There is no native shader cache; render-time allocation is preferred over unsafe bounds-dependent caching for now.
• Legacy public SkiaSharp compatibility members remain to avoid a broad breaking change.
• No animation scheduler or Shape controls are included.

Next steps

1. Harden the shared UI animation scheduler so mutable brush values can be animated without a second timing system.
2. Add the ThemeManager and its renderer-neutral JSON brush converter.
3. Build Shape controls on the common brush contract after the animation and theme foundations are stable.