Additional isolation targets

Broaden virrun's isolation surface beyond the shipped Linux/WSL os backend: a macOS bridge through a lightweight Linux VM, and a Firecracker microVM backend for untrusted multi-tenant / CI fan-out.

Why deferred: The Linux + WSL os backend already covers this repo's dev hosts. Adding more isolation targets is breadth, not depth — and the current focus is making the existing path as fast and as usable as possible, not running on more platforms. A second OS bridge and a microVM backend each carry real ongoing maintenance and a fresh correctness-gate surface for no immediate dev-loop payoff.

Revisit when: The Linux/WSL path's speed + UX goals are met and a concrete consumer needs it — a macOS dev host with no WSL equivalent, or an untrusted multi-tenant / CI fan-out workload that the rootless bubblewrap sandbox can't safely host.

Deferred

Ideas virrun chose not to build yet — each with the concrete trigger that would revisit it.

Native task-cache recording

Let the native backend produce task-cache entries, so skip-unchanged-builds works without the os sandbox.