fix(embedding): prevent Intel macOS shutdown crash

[EyJunge1]

Summary

• pin onnxruntime-node to 1.20.1, the last release that includes macOS Intel binaries without the shutdown teardown regression
• discover the pinned package's N-API layout instead of hardcoding it
• extend the compiled-host regression fixture to run real inference and verify natural process shutdown
• document the upstream binary and teardown constraints for Intel macOS

Verification

• bun run typecheck
• bun test
• bun run build
• embedding backend smoke test
• nested compiled-host fixture with real inference and natural exit

Closes #225

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