`ubuntu-latest` moves to 26.04 between 19 October and 19 November

Ubuntu 26.04 is GA for GitHub Actions on x64 and arm64, and the ubuntu-latest label migrates off 24.04 gradually across that month-long window. GitHub says the image removes tools and changes tool versions, and that the move may break builds depending on either. Two options and a deadline: test against runs-on: ubuntu-26.04 now, or pin ubuntu-24.04 and decide later.⁵