Publish Core as an independent companion package

[vpapenko]

Summary

• publish the platform-independent code as ColorPicker.Maui.Core while keeping normal development connected through a direct project reference
• preserve binary compatibility with type forwarders from ColorPicker.dll
• publish coherent preview pairs to GitHub Packages and test both direct and transitive package consumption
• add a GitHub Actions Run workflow form for independent Picker, Core, or coordinated stable releases

Versioning and release behavior

• stable Picker and Core versions are independent
• package builds compile Picker against the actual Core nupkg
• coordinated releases publish and restore Core before publishing Picker
• retries preflight package contents and package-specific tag collisions before any irreversible publish
• GitHub releases use picker-v* and core-v* tags

Validation

• 1,369 Core tests
• Windows and Android library builds
• paired, Core-only, and Picker-only packing
• direct Core, transitive Picker, and type-forwarder package smokes
• GitHub Actions lint plus Bash and PowerShell script parsing