chore: define MSRV and reproducible build policy

[tappe9]

Closes #36.

Summary

• verify and declare Rust 1.85.0 as the workspace MSRV
• commit the root Cargo.lock for reproducible workspace, CLI, CI, and release builds
• require locked dependency resolution in CI
• add a dedicated MSRV check/test job alongside stable quality gates
• document the MSRV and lockfile policy in CONTRIBUTING.md

MSRV verification

Rust 1.85.0 was tested directly in GitHub Actions before declaring it. The probe generated the dependency graph using Cargo 1.85.0, which selected the latest Rust 1.85.0-compatible versions, then successfully ran:

cargo check --workspace --all-targets --all-features --locked
cargo test --workspace --all-features --locked

Rust 1.85.0 is also the compiler floor for Edition 2024.

Validation

The final PR commit passes:

• MSRV (Rust 1.85.0)
• Quality gates
  • format
  • clippy with warnings denied
  • all-feature tests
  • rustdoc with warnings denied
  • no-default-features core tests
  • package contents validation
  • publish dry run
• Tests (ubuntu-latest)
• Tests (windows-latest)
• Tests (macos-latest)