maint: replace the styling and Linear research with StyleX

[karngyan]

Summary

• replace the stale Linear audit with authenticated current UI research
• define a precompiled StyleX package architecture with no consumer setup
• expand the target catalog to 113 components, including product-grade composites
• supersede the old plain CSS design and implementation plans
• update repository guidance for the StyleX-only direction

Validation

• pnpm format:check
• git diff --check

Notes

This is a research and architecture change only. It does not migrate package code yet, and it intentionally requires no backward compatibility because Dowel has not been released.

[karngyan]

Closing because development will continue through direct commits to main as requested.