## Problem
A macOS runner on #206 failed the **Rust** job's clippy step with:
```
Updating crates.io index
error: failed to get `rayon` as a dependency ...
download of config.json failed
[6] Couldn't resolve host name (Could not resolve host: index.crates.io)
```
A pure transient DNS blip — unrelated to the change (a docs-only PR). `CARGO_NET_RETRY=10` only does fast in-process retries; a longer DNS outage outlasts them, so the very first cargo step's crates.io index fetch fails the whole job.
## Fix
Add a **Warm cargo registry** step right after the cache restore in the `rust`, `clippy-bindings` and `msrv` jobs. It runs `cargo fetch` in a 5-attempt loop with real backoff (20/40/60/80s sleeps) so the dependency graph is pulled once, patiently, riding out a multi-second DNS outage that cargo's rapid retries can't. The later clippy/build/test steps then resolve from the warmed local cache.
Mirrors the existing inline retry pattern already used for setup-node/setup-python CDN flakes. Can be extended to the coverage/python/wasm/node jobs if they ever hit the same blip.
kingchenc
d16df1e224