feat: add WindowProvider, PlatformProvider; remove legacy TouchEventSource

[kolkov]

Summary

• Add WindowProvider interface — window geometry (Size), DPI (ScaleFactor), and RequestRedraw for on-demand rendering
• Add PlatformProvider interface — clipboard read/write, cursor management, dark mode, accessibility preferences (reduce motion, high contrast, font scale)
• Add CursorShape enum with 12 standard cursor shapes and String() method
• Add NullWindowProvider / NullPlatformProvider for testing and headless scenarios
• Remove TouchEventSource — legacy interface fully replaced by W3C Pointer Events Level 3 (PointerEventSource with PointerType: Touch). Zero implementations, zero consumers existed
• Update public documentation (README, CHANGELOG, ROADMAP)

Design

• WindowProvider is a standalone interface (3 methods)
• PlatformProvider is optional — check via type assertion: if pp, ok := provider.(PlatformProvider); ok { ... }
• CursorShape lives in gpucontext to avoid circular dependency with ui
• Enables gogpu/ui to access host window and platform capabilities without direct gogpu dependency

Test plan

• 56 tests pass (go test ./...)
• go vet clean
• golangci-lint 0 issues
• go fmt clean
• CI validates cross-platform (Linux, macOS, Windows)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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