Preserve configuration during interrupted saves

[lukisch]

Summary

• Save configuration through a closed sibling temporary file and atomically replace the target, preserving the previous bytes when write or replace fails.
• Add synthetic regressions for partial temporary writes, replace errors, and preserved error reporting.
• Declare Pillow and the test tools in requirements-dev.txt, use it in CI and contributor setup, and provide the tomli fallback required by the Python 3.10 test matrix.

Validation

• python -B -m pytest -q -p no:cacheprovider — 98 passed, 6 subtests passed (Python 3.12.10; isolated scratch profile/temp).
• ruff check . — passed.
• python -B -m compileall -q . — passed.
• git diff --check — passed.