fix: polish native Windows paths

[raoabinav]

Evaluator Summary

• Abi fixed native Windows portability issues in LEANN's chat and local-model integration paths, improving behavior outside Unix-like shells.
• Design choice: the patch stays narrow and uses platform-native abstractions, replacing POSIX-only assumptions with os.pathsep for NODE_PATH and guarded timeout behavior where SIGALRM is unavailable.
• The implementation also makes the CPU mkldnn.enabled setting type-checker-friendly, reducing platform-specific lint/type noise in embedding setup.
• Quality bar: focused Windows and LM Studio tests passed locally, plus lint, format, type, lockfile, and whitespace checks; this is a clean supporting PR rather than a flagship roadmap feature.