docs(bootstrapper): fix docstrings and add test for write-error logging

[dhellmann]

This PR has a few changes from Claude's review feedback on #1249

• Correct add_to_build_order docstring: build-order data is now buffered and written once in finalize(), not on each call
• Update finalize() docstring to note that background I/O errors are logged but not raised
• Add comment to __exit__ explaining pending writes are intentionally abandoned on error exit
• Add "Must only be called from the main thread" note to _write_graph_async docstring
• Add comment to test_record_stack_state_throttled_when_called_rapidly warning about st_mtime filesystem-resolution dependency
• Add test_check_write_error_logs_on_failure to verify that failed background write futures are logged rather than raised

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📥 Commits

Reviewing files that changed from the base of the PR and between 4c18163 and 12315ee.

📒 Files selected for processing (2)

• src/fromager/bootstrapper/_bootstrapper.py
• tests/test_bootstrapper.py

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