Add a weekly canary job that tries the latest nightly toolchain #22

Context: rust-toolchain.toml is pinned to nightly-2025-07-11. Drift is inevitable. A non-blocking canary gives us a heads-up before a forced bump turns painful.

Scope:

• New scheduled workflow (weekly) that overrides the toolchain to nightly and runs just check + just test.
• On failure, opens (or updates) a tracking issue titled "nightly canary failing".

Acceptance: workflow runs weekly; issue updates visible.