ci: split Rust checks into focused jobs

[ubugeeei]

Problem

The CI workflow currently reports most Rust validation under one rust check. That makes failures less scannable and makes it harder to see which validation layer failed at a glance.

Acceptance criteria

• Split the Rust CI workflow into focused jobs for metadata, formatting, build checks, clippy, and tests.
• Keep stale-run cancellation limited to pull requests.
• Keep the benchmark job manual-only.
• Mirror the split in local scripts so the same checks can be run before using Actions.