Adds `repro.ts`, a self-contained script that:
- Generates an HTML entrypoint, a main TS entrypoint, and thousands of dynamically imported chunks.
- Runs `bun build --splitting` against the generated structure.
- Parses the output `index.html` to confirm whether the injected script matches the main entrypoint or a random chunk.
- Updates `.gitignore` to safely ignore the temporary artifacts (`bun-repro-dir`).
ruinivist