Fix React embed ESM packaging for Vite 8

[byrnehollander]

What changes

• Adds ESM entrypoints for @formsort/react-embed and its direct embed dependency chain while preserving CJS entrypoints.
• Rewrites @formsort/react-embed around hooks, stable event callbacks, cleanup, StrictMode behavior, and prop-driven reloads.
• Handles old CJS-only messaging manager installs from the web ESM entry, and bundles constants into the manager ESM build.

Why

A Vite 8 upgrade exposed that React embed and its dependency chain were still CJS-oriented, which caused the embed to break in modern ESM builds. This PR fixes that packaging issue for other consumers while also modernizing the small React wrapper code that was already being touched.

Upgrade path

This should be a minor upgrade for documented usage: root imports continue to work for ESM and CJS, and React embed props are not renamed or removed. It is minor rather than patch because package output changed and EmbedFlow now reloads when load props such as clientLabel, flowLabel, variantLabel, queryParams, or embedConfig change. Deep lib/* imports are internal and are not guaranteed.

Verification

• corepack yarn build, corepack yarn test, corepack yarn lint
• Focused React embed and web embed API builds/tests
• CJS require and ESM import smoke tests
• Vite 8 packed-tarball build, fake CJS messaging manager smoke, and Parcel event-log example build against packed tarballs

Closes #184