chore(test): migrate from Enzyme to React Testing Library (#249)

* chore(test): migrate from Enzyme to React Testing Library

Replace enzyme and enzyme-adapter-react-16 with @testing-library/react,
@testing-library/jest-dom, and @testing-library/user-event for React 18
compatibility.

- Update jest.config.js to use setupFilesAfterEnv with RTL setup
- Migrate all existing tests to RTL idioms (render, container queries)
- Add additional test coverage for axis restrictions, constraints,
  callbacks, handle directions, event handling, and state management
- Update react-test-renderer to v18 for snapshot compatibility

All 45 tests pass with 98%+ coverage.

Co-authored-by: Sculptor <sculptor@imbue.com>

* docs: add CLAUDE.md with project instructions

Co-authored-by: Sculptor <sculptor@imbue.com>

---------

Co-authored-by: Samuel Reed <foo@bar.com>
Co-authored-by: Sculptor <sculptor@imbue.com>

Author: 3 people

CLAUDE.md

@@ -0,0 +1,71 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## Commands
+
+```bash
+# Install dependencies
+yarn
+
+# Run tests with coverage
+yarn test
+
+# Run tests in watch mode
+yarn unit
+
+# Lint (ESLint + Flow)
+yarn lint
+
+# Type check only
+yarn flow
+
+# Build (transpile lib/ to build/)
+yarn build
+
+# Run dev server with examples
+yarn dev
+
+# Build examples for production
+yarn build-example
+```
+
+Run a single test file:
+```bash
+npx jest __tests__/Resizable.test.js
+```
+
+Run tests matching a pattern:
+```bash
+npx jest --testNamePattern="snapshot"
+```
+
+## Architecture
+
+This is a React component library providing resizable functionality via two main components:
+
+### Core Components (`lib/`)
+
+- **Resizable.js** - Stateless base component. Wraps a child element with draggable resize handles using `react-draggable`'s `DraggableCore`. Computes size changes from drag deltas, applies constraints, and invokes callbacks. Does not manage state - parent must set `width`/`height` props from callback data.
+
+- **ResizableBox.js** - Stateful wrapper around `<Resizable>`. Manages width/height state internally and renders a `<div>` with those dimensions. Simpler API for common use cases.
+
+- **propTypes.js** - Shared Flow types and PropTypes definitions. Exports `resizableProps` object and types like `ResizeHandleAxis`, `ResizeCallbackData`, etc.
+
+- **utils.js** - Helper `cloneElement()` that merges `style` and `className` when cloning React elements.
+
+### Key Implementation Details
+
+- Resize handles are rendered as `<DraggableCore>` wrappers around handle elements
+- Handle positions: `'s'`, `'w'`, `'e'`, `'n'`, `'sw'`, `'nw'`, `'se'`, `'ne'`
+- The `runConstraints()` method applies min/max constraints and aspect ratio locking with slack tracking
+- Position tracking via `lastHandleRect` compensates for element repositioning during north/west drags
+- `transformScale` prop adjusts deltas when parent has CSS transform scaling
+
+### Build Output
+
+`yarn build` transpiles `lib/*.js` to `build/` and copies source files as `*.js.flow` for Flow consumers.
+
+## Testing
+
+Tests use Jest with Enzyme for shallow/mount rendering. Test files in `__tests__/` mirror the lib structure. Snapshot tests verify render output; unit tests verify resize behavior, constraint handling, and callback data.