[GEN-10] No `rust-toolchain.toml` — local builds float from CI's `stable`-of-the-day

[intendednull]

Audit finding from #300 (commit 679f9fe)

Severity: low
Category: ci / dev experience
File: .github/workflows/ci.yml:21
Obvious fix: yes

Description

All five CI jobs use dtolnay/rust-toolchain@... # stable. There is no rust-toolchain.toml at the repo root, so a contributor's local toolchain (whatever their default-host is) can drift from CI's stable-of-the-day, especially for clippy lint deltas. CLAUDE.md mandates "zero warnings" before commit, which a local nightly will silently violate.

Impact / Threat

Developer-facing — flaky local-vs-CI clippy mismatches.

Suggested fix

Add a rust-toolchain.toml pinning channel = "stable" (or a specific 1.NN.0) and the wasm32-unknown-unknown target so local builds match CI.

Verify

ls /home/user/willow/rust-toolchain.toml