ci: pin the fmt-check nightly toolchain to a fixed date

[nh13]

The CI fmt check installs a floating nightly toolchain (dtolnay/rust-toolchain with toolchain: nightly), so whenever a new nightly rustfmt is released it silently reformats unchanged code and turns the fmt check red — on every open PR at once, regardless of what that PR changed.

This bit the repo twice in one day: it forced a reformat in #59 (an unrelated spectrum.rs hunk), and then turned the entire open Design-Z PR stack (#54–#57) red on fmt the moment their branches were re-pushed, even though none of them touched the flagged code.

Pin the fmt toolchain to nightly-2026-06-22 — the exact version main and all current branches are formatted against — so fmt drift only ever happens on a deliberate bump of this one line, not on an upstream nightly release. The action itself stays pinned by commit hash as before; only the floating toolchain: value is fixed.

Bumping rustfmt in future is then a one-line, reviewable change (update the date, run cargo +nightly fmt, commit the reformat together) instead of a surprise red on unrelated PRs.

Summary by CodeRabbit

• Chores
  • Standardized formatting checks to use a fixed Rust nightly version, making CI results more consistent and predictable.

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No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro

Run ID: 8f8269db-5761-4179-8311-bc34b49497a5

📥 Commits

Reviewing files that changed from the base of the PR and between 4cc06b9 and 781ebad.

📒 Files selected for processing (1)

• .github/workflows/ci.yml

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Walkthrough

Pins the nightly Rust toolchain in CI from a floating nightly to nightly-2026-06-22 in both the toolchain install step and the cargo fmt invocation within .github/workflows/ci.yml.

Changes

Nightly Toolchain Pin

Layer / File(s)	Summary
Nightly toolchain pin and fmt command .github/workflows/ci.yml	toolchain: nightly replaced with toolchain: nightly-2026-06-22; cargo +nightly fmt updated to cargo +nightly-2026-06-22 fmt to match.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately describes the main CI change: pinning the rustfmt check to a fixed nightly date.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/pin-fmt-nightly

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[nh13]

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