ci: MSRV job + cargo-deny check in Actions

[rmems]

Summary

Add continuous MSRV verification and cargo-deny in CI (deny.toml already present).

Plan

1. Declare rust-version / document MSRV
2. CI job on MSRV toolchain
3. cargo deny check in workflow

Relationships

• Parent: Epic: plasticity-lab v0.2 — Hardening & honesty #43

Acceptance criteria (shared)

• cargo fmt --check && cargo clippy --all-targets --all-features -- -D warnings && cargo test --all-features green

[linear-code]

LIM-1004