App Router crash: reactfire 4.2.4/4.2.5 throw "dynamic usage of require is not supported" on the client #759

Summary

Reactfire 4.2.4 and 4.2.5 (current latest) crash on the client in Next.js App Router apps that render a data hook (e.g. useFirestoreDocData). use-sync-external-store/shim (a CJS module) is bundled into our ESM dist, which makes the bundler emit a dynamic require(). In a strict-ESM browser context it throws at module evaluation of the reactfire chunk:

Error: dynamic usage of require is not supported

Any App Router page that loads a reactfire data-hook chunk breaks on the client. Server prerender is unaffected (Node has require).

Affected versions

• 4.2.4, 4.2.5: affected (ship the bundled shim with the dynamic require).
• 4.2.3: clean (predates the current build toolchain; ESM dist imports react directly, no require).

Reproduction

1. Next.js App Router app on reactfire 4.2.5.
2. Render a client component using a data hook, e.g. useFirestoreDocData.
3. Load the page: the browser console throws Error: dynamic usage of require is not supported at module evaluation.

Root cause

The build-tooling migration (around 4.2.4) began bundling the CJS use-sync-external-store/shim into the ESM output, producing a runtime require() that does not exist in strict ESM.

Fix

Externalize use-sync-external-store/shim so the consumer's bundler resolves it (it is already a runtime dependency). Non-breaking; the shim's React 16/17 fallback is preserved. Fix PR incoming.