Bun mock.module does not intercept node: builtins (1.4.2)

On Bun 1.4.2, mock.module("node:child_process", …) does not change what import { spawnSync } from "node:child_process" returns in later importers — builtin modules bypass the module mock registry. Verified empirically; docs describe mock.module for user modules.

Proven substitutes:

• PATH shim: create a fixture-local executable script (shebang #!/path/to/bun), set mode: 0o755, and prepend its directory to PATH. Capture argv/stdout to a JSON file; assert on the file. Works for real spawnSync and any child that resolves commands via PATH. Combine with PATH=<shim-dir> alone (no system dirs) to prove no other executable can run.
• spyOn before import: for process.*/globalThis seams (process.kill, fetch, Bun.serve), spyOn(...).mockImplementation(...) placed before dynamically importing the module under test works.

Why: teams burn time on the documented-looking mock.module path for builtins and silently get the real implementation (worse: real subprocesses). How to apply: in Bun CLI tests that must fake subprocesses, default to a PATH shim recorder; keep module mocks confined to a fresh child process so spy state never leaks across cases.