CI: target/ is in the cache paths but never actually persists any Anvil build #26

Created by Claude.

Every job in .github/workflows/ci.yml caches the same paths under the same key:

path: |
  ~/.cargo
  ~/.rustup
  ~/verus
  rust-toolchain.toml
  target
key: toolchain-${{ runner.os }}-${{ needs.setup-toolchain.outputs.verus-sha }}

Nothing ever puts an Anvil build into that target/. Only setup-toolchain writes the cache, and it builds Verus into $HOME/verus; it never builds Anvil. Every other job restores the same key, so its post-step finds the key present and saves nothing: Cache hit occurred on the primary key toolchain-Linux-…, not saving cache.

Each job therefore compiles Anvil from scratch and throws the result away. That is most of the 4m30s–5m13s each e2e job spends deploying its controller, and the ~2 minutes full-verification spends compiling before it verifies anything.

Fix

Split the one key into two caches with different lifecycles:

• Toolchain — ~/verus, ~/.rustup, rust-toolchain.toml, keyed on the Verus revision. This cache is immutable, so downstream jobs should use actions/cache/restore@v6 with fail-on-cache-miss: true.
• Build — ~/.cargo/registry, ~/.cargo/git, target/, keyed on the Verus revision plus a source hash, with a restore-keys: prefix fallback so a source change still restores a warm target/ and rebuilds only what changed. Swatinem/rust-cache handles that key management.

rust-toolchain.toml is generated by tools/setup-verus.sh and sits in the cache paths, so a restore overwrites the checked-out copy. Keeping it in the toolchain cache is fine, but be deliberate about it.