feat(perf): add reusable perf-matrix.sh test harness + iperf3 instrumentation

perf-matrix.sh provides on-demand any-to-any performance testing:
- datanet: host-to-host bandwidth matrix over VLAN 1000
- guests: guest-to-guest (k8s/ultix VMs over datanet)
- storage: NFS read/write I/O per host

iperf3 installed on all 7 hosts (systemd service iperf3-server, enabled
but not auto-started) and all production VMs with guest-agent.

Also: tsys3 datanet persistence fix (USB NIC late-enumerate systemd
service), 4 missing reverse DNS zones created, netbird/ultix-mini/
ultix-sidecar/ultix-highside DNS records added (DNS now fully
consistent across all Tailscale nodes). [#413]

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WORKING.md

@@ -5,7 +5,6 @@ A commit is blocked while any task below remains unchecked.
 
 ## Current Tasks
 
-- [x] Switch rename Phase 4-5: mapping.txt + conman/ser2net/udev + repo files
-- [x] End-to-end perf testing (#413): host-to-host, storage, guest-to-guest
-- [x] Fix tsys3 datanet (physical NIC not enslaved to bridge)
-- [x] Kuma monitor fix (DNS aliases for old switch names)
+- [x] Install iperf3 on all hosts + production VMs (persistent)
+- [x] Create reusable any-to-any perf test script
+- [x] Enable iperf3 systemd service on hosts for on-demand testing