## Summary
- When an absolute path prefix is provided to `temp.mkdir()` or
`temp.mkdirSync()`, use it directly instead of joining with
`os.tmpdir()`
- This supports use cases where compilers need temporary directories in
specific locations (e.g., shared NFS drives for remote compilation)
- Existing callers all use relative prefixes, so this is fully backwards
compatible
## Test plan
- [x] Added tests for both `mkdir` and `mkdirSync` with absolute paths
- [x] Verified existing tests still pass
- [x] Checked all callers use relative paths (no behaviour change)
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Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>
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