Fix save failures caused by invalid filename characters

[LingYi-Liang]

Fixes #106.

Text such as 2026/10/03 produces path separators in the generated filename, so saving fails even when the output directory exists and is writable.

Replace path separators, Windows-invalid characters and ASCII control characters in the text fragment with underscores. Unicode text (including Chinese), the existing first-10-character truncation, and full video task IDs are preserved.

The same replacement rules apply on all platforms, so : and ? are also replaced on systems such as Linux where those characters are allowed.

Added 30 filename regression cases that write files in temporary directories. On Windows with Python 3.11.9, all 36 tests pass, including the existing six utility tests; the original code fails 22. No model API calls were made.

Run:

python -m pytest -q -o addopts= tests/test_output_filenames.py tests/test_utils.py

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