ci: establish Minimum Supported Rust Version (MSRV) policy and test it in CI #14

Summary

Declare an MSRV (Minimum Supported Rust Version — the oldest Rust toolchain version we guarantee the crate will build + pass tests on) for engram-parser (e.g. the lowest stable that builds clean with current features), document it in Cargo.toml (rust-version) + README, and add explicit CI job/matrix entry testing on that toolchain (fmt/clippy/test --all-features) alongside stable. Follow common Rust lib practice and corinth patterns where relevant. (User note: IDK what is MSRV — definition added here.)

Motivation

• Reusable library users need to know what Rust they can use.
• Prevents accidental use of newer-only features.
• CI on MSRV catches regressions early (like the cross-platform in Add Azure Pipelines for cross-platform CI verification (macOS, Windows, Linux) #8).
• Good DX for Limen-Neural consumers (cortex etc).

Scope

• Choose/verify MSRV (use cargo-msrv or manual; current edition="2024" implies recent, but pin e.g. 1.80+ or whatever works).
• Add rust-version = "1.XX" to [package] in Cargo.toml.
• Update ci.yml (or harden issue) to include a "msrv" job: dtolnay/rust-toolchain with toolchain: 1.XX , then cargo check/test etc.
• README: "MSRV: 1.XX (tested in CI on stable + MSRV)".
• Perhaps clippy on MSRV too (with allowances if needed).
• Cross links.

Non-goals

• Testing beta/nightly in main CI (optional separate).
• MSRV bumps in same PR as features (separate).
• Polyfill or compat shims.

Acceptance Criteria

• rust-version set in Cargo.toml.
• CI has passing MSRV job (on every PR/push).
• README + perhaps lib.rs docs updated with MSRV.
• Cross to Add Azure Pipelines for cross-platform CI verification (macOS, Windows, Linux) #8 (Azure should also consider MSRV) and harden issue.
• Policy note: how/when we bump MSRV (major or with justification).

Open Questions

• What is the actual lowest MSRV today (run discovery)? (User: "I still don't know how to answer that btw")
• Should Azure (Add Azure Pipelines for cross-platform CI verification (macOS, Windows, Linux) #8) also matrix the MSRV? (Matrix and cross platform verification. But this azure issue has already been made — so covered by existing Add Azure Pipelines for cross-platform CI verification (macOS, Windows, Linux) #8.)

Related

• Add Azure Pipelines for cross-platform CI verification (macOS, Windows, Linux) #8 Azure (cross-platform + MSRV synergy)
• Harden GH CI issue
• Cargo rust-version docs
• corinth (if it declares MSRV)

Standard for publishable Rust crates under Limen-Neural.

Drafted and planned with assistance from Grok Build (xAI) for Limen-Neural/engram-parser (2026-07-03). Cite as Grok Build when creating.