feat(react): pilot AffineScript port of Button per migration playbook

Step (c) of the TS-elimination plan. First .affine source in this repo.

What's done:
- Button.affine — full re-decomposition of Button.tsx per the playbook's
  Cardinal Rule ("re-decompose, don't transliterate"). Concrete moves:
    * forwardRef class → standalone make_button fn returning own Element
    * React 'a | b | c' string unions → ButtonVariant/ButtonSize sum types
    * HTMLButtonAttrs spread escape hatch → dropped (caller mutates returned own Element)
    * iconBefore?: ReactNode → Option[own Element]
    * Defaults via default_button_props() factory
    * DOM mutations declared `/ IO` until @affinescript/dom publishes a typed DOM effect
    * Inline DOM extern declarations (replace with @affinescript/dom import once
      cross-package import story is settled in this repo)
- Index.affine — version + wcag_level constants + PORTING_NOTES block
  documenting the Modal decomposition strategy for the next session.
- Button/Button.tsx removed
- index.ts removed

What's not done (deferred):
- Modal.tsx left in place. Its 5 useEffect blocks (portal mount, focus trap,
  escape-key, scroll lock, focus restoration) each become a separate own-
  resource with Drop semantics in AffineScript — designed but not written.
  See PORTING_NOTES in Index.affine.
- package.json still references rollup + jest + @reach/auto-id; build pipeline
  swap to affinescript-vite is a separate concern, not blocking this pilot.

Reference: hyperpolymath/affinescript:docs/guides/migration-playbook.adoc

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath