rust-toolchain.toml pinned the floating "stable" channel, so the CI image baked whatever
stable existed at image-build time. When the image is rebuilt onto a newer stable,
rustfmt's rules shift and `cargo fmt --all --check` fails on files nobody touched — the
recurring format-drift that keeps red-lighting CI.
Pin channel = "1.96.0" (== today's stable: rustc ac68faa20, rustfmt 1.9.0-stable), the
exact build CI already runs, so this is a no-op now but locks formatting for good: local
dev, the Linux CI image, and the Windows runner all use rustup and honor this file, so
they converge on one rustfmt. Formatting now only changes in a deliberate bump-this-pin-
and-reformat commit. ci.yml cache comment updated to match.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
enricobuehler
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