Handle stable warning snapshots

[tdashelby-cmyk]

This addresses #498.

The warning tests compare trybuild stderr fixtures. The existing fixtures match the nightly diagnostic rendering used by the repository CI, while the Debian report shows the stable-style rendering for three diagnostics where help notes are printed before the source location.

This keeps the current nightly warning run unchanged and adds stable-specific fixtures for those three cases. On nightly, the test still runs tests/warnings/*.rs; on non-nightly, it runs the unaffected warning cases plus the stable fixtures.

Verification:

• git diff --check
• git show --check --oneline HEAD
• Compared the stable fixtures against the actual output in Debian: Failing tests #498

I could not run cargo test -p maud --test errors run_warnings locally in this environment because cargo is not installed on PATH. The first CI run showed the original fixture replacement was wrong for nightly, so I amended this to keep the nightly fixtures unchanged and add non-nightly fixtures instead.

[tdashelby-cmyk]

CI reran after the branch update. The warning-test job (Main) is now passing, along with Documentation and Rustfmt.

The remaining red job is Doctest; it fails while compiling rustix 0.37.28 with cannot find attribute rustc_layout_scalar_valid_range_*, before reaching this change's warning tests.