The Rust Workspace Tests job manually cached the whole `v2/target` via
actions/cache@v4. For a 38-crate workspace that dir is multi-GB, and several
CI runs this cycle intermittently died at the cache/setup step (after
toolchain install, before "Run Rust tests"), each needing a rerun.
Swatinem/rust-cache@v2 is the de-facto standard Rust CI cache: it caches the
cargo registry/git + a pruned target, evicts stale dependencies, and restores
large workspaces far more reliably and faster than a naive whole-target cache.
`workspaces: v2` points it at the v2/ cargo workspace.
Reliability/speed change — verified by observing subsequent main runs.
Co-Authored-By: claude-flow <ruv@ruv.net>
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