Honor custom comma in threshold number formatting

[gaoflow]

Summary

• use the supplied comma separator when threshold formatting keeps numbers numeric
• preserve comma=0 behavior that disables digit grouping
• add regression coverage for custom separators across one and multiple groups

Fixes #176

Verification

• python -m pytest tests/test_numwords.py::test_lines -q
• python -m pytest -q
• python -m compileall inflect tests
• python -m ruff format --check inflect/init.py tests/test_numwords.py
• git diff --check upstream/main...HEAD

Note: python -m ruff check inflect/init.py tests/test_numwords.py still reports pre-existing UP006/UP007/UP035 typing modernization diagnostics across inflect/init.py that are unrelated to this change.