Dev: keep focused test builds fast and cache-bounded

[chrisbbreuer]

Parent: #50
Related: #52

Problem

Repeated focused ReleaseFast test filters currently relink the monolithic root test artifact under distinct cache keys. During baseline-JIT development, the repository-local .zig-cache grew to 92 GB and LLVM failed with No space left on device. Clearing only the reproducible cache recovered 92 GB. This makes the intended focused-test workflow unnecessarily slow and unsafe for developer disks.

Scope

• provide small focused test artifacts/steps for VM, JIT, benchmark harness, and concurrency primitives instead of pulling every root module test into every filter;
• measure cache growth across representative repeated filtered builds;
• add an explicit documented cache-inspection/cleanup path that cannot delete source or user-owned files;
• keep the full root suite as the batched integration gate.

Acceptance criteria

• JIT/VM focused tests compile and link without the monolithic root test artifact.
• Ten distinct focused test invocations keep repository-local cache growth within a documented reasonable bound.
• CI still runs the complete root suite and focused steps use the same production modules/options.
• Developer documentation explains inspection and safe cleanup of reproducible Zig caches.
• No cleanup command can target paths outside the repository cache/build outputs.

[chrisbbreuer]

Incident detail: repository-local .zig-cache reached 92 GB across distinct monolithic filtered-test artifacts and LLVM failed at 117 MB free. Reproducible Zig caches/build outputs were cleared across ~/Code/Libraries/zig-*; clean stale worktrees were removed, dirty worktree edits were preserved in named stashes before removal, and free space recovered to 108 GB. This is mitigation only; the focused-artifact/cache-bound acceptance work remains open.