ci(perf): cache the bashunit download in CI

[Chemaclass]

Context

tests.yml installs bashunit every run via bash install-dependencies.sh (re-downloads the pinned version). No actions/cache — verified.

Scope

• Add actions/cache keyed on the pinned bashunit version (from install-dependencies.sh / BASHUNIT_VERSION) around lib/.
• Cache miss falls back to the existing install step.

Boundary

Files: .github/workflows/tests.yml only. Coordinate with the workflow-consolidation issue (#TBD) — do whichever merges first, then rebase the other.

Acceptance criteria

• Warm CI run restores bashunit from cache (no re-download)
• Cache key changes when the pinned version bumps
• Tests still green on both ubuntu and macos

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Constraints (apply to all bashdep work)

• Bash 3.2+ compatible — no declare -A, ${var,,}/${var^^}, mapfile, negative array index, &>>.
• Zero runtime deps beyond curl/wget/awk/mktemp/mkdir/rm/cp/mv/printf/cat/grep/sort/tr/dirname/basename + shell builtins.
• TDD: write the failing test first (tests/unit/*_test.sh), then implement; keep make test green for the right reason. Mock curl/wget — no network in tests.
• Quality gate: make test && make sa && make lint must all pass.
• Update docs/api.md / docs/behavior.md when public API or semantics change; add a CHANGELOG.md ## [Unreleased] entry for any user-visible change.
• Conventional commits (ref: for refactors); never mention AI/automation. One PR per issue, assigned to Chemaclass.