Use portable date formatting in stats banner

[ChimdumebiNebolisa]

Summary

Replace glibc-only %-d strftime with portable date formatting in the stats banner SVG.

Problem

generate_banner.py used strftime("%B %-d, %Y"), which raises ValueError on Windows. CI passes on Linux, but local banner regeneration on Windows fails silently behind a warning.

Changes

• Add format_banner_date helper using dt.day
• Allow optional today in svg() for deterministic tests
• Regression tests for formatting and SVG output

Validation

• python -m unittest discover -s .github/scripts -p "test_*.py" - OK (includes BannerDateFormat)

Scope

SVG layout unchanged; only the "as of" date string construction.