perf(dev): share Cargo package cache across worktrees

[johnmatthewtennant]

Summary

Hermit currently gives each Berd worktree its own CARGO_HOME. That duplicates registry and Git dependency downloads and gives dependency sources checkout-specific absolute paths.

Use ${HOME}/.cache/berd/cargo-home as Berd's stable Cargo package home and keep its bin directory on PATH. Cargo's package-cache locking supports concurrent access. Cargo target directories remain independently configured and are not shared by this change.

Verification

Two fresh macOS worktrees activated Hermit against one initially empty shared Cargo home and built concurrently without package-cache errors. Both resolved the same CARGO_HOME; the second worktree reused all 59 dependencies in focused cargo check and cargo test --no-run runs with no downloads.

[morgmart]

🤖 Automated code review

APPROVE: Engineering review of the exact prepared three-dot comparison found no publishable findings. The shared Cargo home and binary path are configured together, and the changed test protects both configuration contracts. Supplied GitHub evidence is structurally valid and shows all 10 check runs completed successfully at the reviewed head SHA; required checks still govern merge readiness.

Deterministic publication result: 0 blocking and 0 non-blocking inline finding(s) publishable; 0 duplicate(s) suppressed; 0 blocking screenshot-evidence requirement(s) in this review body.

Pending checks: 1 check(s) are not complete.

This approval reflects the completed code review only; merge readiness remains governed by the repository's required checks.