chore(crate): declare and CI-test an MSRV

[BryanFRD]

Package
Which package does this affect? [ ] cli [ ] schema [ ] docs [x] ci

Problem / Motivation
Cargo.toml has no rust-version field. rust-toolchain.toml pins nightly. For a crate published to crates.io and used via cargo install, declaring an MSRV (and adding a stable-toolchain CI job to verify it) is standard hygiene.

Without an MSRV:

• Users on stable can't tell whether their toolchain is supported.
• Accidental use of a newly-stabilised feature can silently break older installs.

Proposed solution
Acceptance criteria

• Pick an MSRV target (likely 1.85+ given edition = "2024").
• Add rust-version = "1.XX" to Cargo.toml.
• Add a cargo +1.XX check --workspace job in CI that runs alongside the nightly job.
• Document the MSRV bump policy in CONTRIBUTING.md (one-version-per-minor cadence is standard).

Alternatives considered
None considered explicitly — this is the natural shape.

Additional context
Severity: P2

Category: oss-hygiene

Kit / UI candidate: No.