RFC: table column resize via CSS variables

[coolassassin]

RFC: apply table column widths via CSS custom properties during drag-resize, so React state and virtualizer relayout run once on endResize instead of on every pointer move.

Proof-of-concept implementation: #10267

This PR contains only rfcs/2026-table-column-resize-css-vars/README.md

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices
• I understand every change in this PR and can explain why it's there.
• If AI-assisted, I followed our AI contribution guidance and pointed my assistant at CLAUDE.md.

📝 Test Instructions:

RFC-only PR — no runtime code in this change.

1. Read rfcs/2026-table-column-resize-css-vars/README.md.
2. Discuss the open questions in the RFC (Virtualizer extension point, onResize during drag, whether CSS variable helpers should be public).
3. Optionally compare with the proof-of-concept implementation in feat(table): apply column widths via CSS variables during resize #10267 (Spectrum TableView / RAC ResizableTableContainer, mouse and keyboard resize).

🧢 Your Project:

Devexperts internal projects

POC implementation:

#10267