Give a clear error when a board has no export

[posthog]

Problem

• A first-time user in the tscircuit.com editor writes a bare <board>...</board> with no export, and gets React's raw Element type is invalid ... but got: object — a pointer at React internals, not the one-line fix they need.
• A file with no imports or exports makes Sucrase emit no __esModule marker, so module.exports stays {}.
• The require interop in lib/eval/eval-compiled-js.ts then hands that empty object back as default. It is truthy, so the entrypoint's (() => null) fallback never fires and the empty object reaches circuit.add.
• The runTscircuitCode.ts guard that catches a missing export keyword only runs for string input, so the editor's fsMap path skips it.
• Small in volume, but it is the exact first-circuit mistake, and the sibling JSX-literal case already gets a friendly message — so this is a gap in diagnostics that already exist.

Changes

• Interop: in eval-compiled-js.ts, return undefined for default when the module object has no own keys, so an empty module no longer reads as a component.
• Entrypoint: the generated entrypoint drops the silent (() => null) fallback and instead checks the resolved component is a function. If not, it throws a tscircuit-worded error that names the file and tells the user to add export default.
• Test: a single no-export index.tsx in the fsMap pins the new error.

Result

Before:

Error evaluating "entrypoint.tsx": Element type is invalid ... but got: object.

After:

Error evaluating "entrypoint.tsx": No component was exported from "index.tsx".
You need to export a component, e.g.

export default () => (
  <board width="10mm" height="10mm" />
)

---

Created with PostHog Desktop from this inbox report.

[github-actions]

This PR has been automatically marked as stale because it has had no recent activity. It will be closed if no further activity occurs.