feat(node): periodic Reaper to GC orphaned VM resources#44
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An unclean VM death (host OOM-kill, `:erlang.halt`, a crash before the FireVMM relaunch-time cleanup runs) can strand a firecracker cgroup leaf and a `hyper-rw-*` thin volume whose vm_id never reboots, so nothing ever clears them. Add `Hyper.Node.Reaper`, a periodic liveness-aware GC started last under `Hyper.Node`. Each tick it lists the cgroup leaves under the parent cgroup and the `hyper-rw-*` dm volumes, subtracts the VMs this node still runs (via the VM supervisor and the routing CRDT), and -- only after a leaf shows up orphaned on two consecutive ticks (`Plan.confirm/2`, so a VM mid-boot is never reaped) -- removes the chroot/cgroup via the helper and the dm volume. Entirely best-effort: every failure is logged and the next tick retries. The decision logic is the pure `Hyper.Node.Reaper.Plan` (rw_ids/orphans/confirm), covered by example + property tests; the GenServer is a thin shell around it. The tick interval is a fixed constant -- the two-strike confirmation already makes the exact value non-load-bearing, so nothing here is configurable. Also corrects `Jailer.cgroup_dir/1` to `<parent>/<id>` (no `<exec>` level): the jailer (cgroup v2, `--parent-cgroup`) places firecracker directly under the parent cgroup, confirmed via `/proc/<pid>/cgroup`. This also fixes the existing FireVMM teardown (`daemon.ex`), which removes the same leaf, and is what lets the Reaper enumerate leaves by listing the parent dir. Extracted from the VM-boot branch (chore/get-a-vm-running).
markovejnovic
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