fix: use sys.stdout.write for thread-safe output in StdoutSink

[jaheyy]

Summary

Fixes #139

Replace print(serialized_content) with sys.stdout.write(serialized_content + "\n" in StdoutSink to prevent interleaved output in multi-threaded environments.

sys.stdout.write(serialized_content + "\n") issues a single write call, which is atomic under CPython's GIL.

Verification

Tested with a repro script:

=== Running repro.py 10 times (output in /tmp/emf_repro.KLRzjP) ===

Run  1: 20000 lines, 0 interleaved
Run  2: 20000 lines, 0 interleaved
Run  3: 20000 lines, 0 interleaved
Run  4: 20000 lines, 0 interleaved
Run  5: 20000 lines, 0 interleaved
Run  6: 20000 lines, 0 interleaved
Run  7: 20000 lines, 0 interleaved
Run  8: 20000 lines, 0 interleaved
Run  9: 20000 lines, 0 interleaved
Run 10: 20000 lines, 0 interleaved

Total interleaved across 10 runs: 0