ci: add clippy and rustfmt gates

[Hollujay]

Summary

CI runs the test suite. It does not currently enforce lint or formatting, so
style drift and common Rust footguns can land without being caught in review.

Acceptance Criteria

• Add cargo clippy --workspace --all-targets -- -D warnings as a CI step
• Add cargo fmt --all -- --check as a CI step
• Fix any existing violations in the same PR so the gate starts green
• Each gate is a separate named step, so a failure is attributable

Tech Stack

GitHub Actions, Rust toolchain