accept iterable inputs in write_lines type annotation

[gheshm-jpg]

write_lines accepts tuples and generators at runtime, but its list[str] annotation makes mypy reject both. the implementation already forwards its input to _replace_linesep, which accepts Iterable[str].

this broadens the public annotation to Iterable[str] and updates the parameter documentation. a typed regression test writes a tuple, appends a generator, and checks newline normalization and the resulting file contents.

validation on macOS with Python 3.13.7:

• a minimal tuple/generator caller produced two mypy argument errors before the change and passes afterward.
• python -m pytest -q: 161 passed, 1 skipped, and 7 ruff failures. the unchanged base reports 160 passed, 1 skipped, and the same 7 ruff failures with Ruff 0.16.6.
• git diff --check passed.

no runtime implementation changes.