test(runtime): restore bun-error fixture for runtime corpus

Tracks a corpus-layout sub-gap from #66.

Home mirrors Bun tests under packages/runtime/test/test, so Bun test imports that climb to repository-level packages/ instead resolve under packages/runtime/test/packages/. js/bun/runtime-error.test.ts therefore could not load packages/bun-error/runtime-error and the scanner misclassified the file as a crash.

Port the exact Bun packages/bun-error/runtime-error.ts fixture into the preserved corpus layout without changing the upstream test import. The fixture has been compared byte-for-byte with live Bun and the focused Home test passes 1/1 under a 768 MB bound.

Fixed on main by a293412f9. The fixture is byte-identical to live Bun, the upstream test import is unchanged, and the focused Home corpus run passes 1/1 with 2 expectations under a 768 MB bound. This scan row was missing corpus support, not a runtime crash.