perf(dev): share Cargo package cache across worktrees

[johnmatthewtennant]

Category: infrastructure
User Impact: Rust dependency builds can now be reused across Berd worktrees instead of being invalidated by checkout-specific package paths.

Problem: Hermit gives each Berd worktree its own CARGO_HOME. That changes the absolute paths of downloaded Rust dependency sources, so Cargo invalidates otherwise reusable compiler artifacts copied or cached from another worktree.

Solution: Berd now uses a stable user-level Cargo home for registry, Git, and package caches while keeping each worktree’s Cargo target directory isolated. Cargo’s package-cache locking supports concurrent access, so dependency downloads are shared without reintroducing build-directory lock contention. The shared Cargo home’s bin directory remains first on PATH, preserving cargo install behavior.

File changes

bin/hermit.hcl
Overrides the Hermit rustup package’s checkout-local Cargo home with ${HOME}/.cache/berd/cargo-home and prepends that Cargo home’s bin directory to PATH.

scripts/release/tests/release-scripts.test.mjs
Pins the shared Cargo-home and binary-path contracts so future Hermit package changes cannot silently restore worktree-specific dependency paths.

Related issue

None found.

Testing

• In two fresh macOS worktrees at the same commit, a seeded cargo test -p berdctl --no-run consumer reported 59 Fresh crates, 0 Dirty, 0 Compiling, and 0 downloads, finishing in 0.15 seconds.
• The same fresh-worktree experiment with cargo check reported 59 Fresh crates and finished in 0.15 seconds. Clippy reused all 58 dependencies and rechecked only the path-sensitive workspace crate.
• Two fresh worktrees built concurrently with separate target directories and one shared empty Cargo home. Both succeeded with no build-directory lock waits; Cargo briefly serialized only its package cache.
• Hermit resolves both the stable Cargo home and its binary path on macOS. The focused release-script regression passes locally; the stable-home contract and just check also passed on Linux Blox.