fix: resolve the code-review findings on the kernel datapath

Compute:
- Track the anchor that owns uBridge's single AF_PACKET capture slot: a
  second kernel port's capture is refused with a clear error (its flag
  rolled back), a failed start no longer wedges the port, and only the
  owning port's stop issues the process-wide `capture stop_kernel`
  (docker/qemu/iou/dynamips/switch, via the mixin's slot helpers).
- Unwind failed starts instead of leaking uBridge and the anchor TAPs:
  QEMU's stop body runs under try/finally, QEMU and IOU start the datapath
  inside the launch try, Dynamips undoes a half-attached port (the retry
  used to be short-circuited into a silent black hole), and Docker deletes
  the veth pair when the namespace move fails.
- Restore the relay filter guard on IOU (kernel-only filters raise
  NodeError, an uncompilable BPF line degrades to a warning), validate the
  eBPF/quota/window/bpf filter shapes before touching the bridge (short or
  non-numeric values surfaced as 500s after partial state), and refuse a
  second NIO on an occupied Docker/QEMU port (the guard IOU and Dynamips
  already had).
- Probe fixes: `_probe_bridge_tap` survives a failing uBridge start, and
  the probe caches key on ctime too, so a chmod/setcap remedy re-probes.

Controller:
- Parse GNS3_UNIX_SOCKET_NIO like the compute does (line-anchored, truthy
  value) and mirror the compute's class selection in kernel_anchor_type.
- Read partial capability reports defensively; make BridgeNIO.bridge
  required-but-nullable so an omitted key is a 422 rather than a KeyError.
- Strip the packet_loss correlation slot even when it is zero: the relay
  filter takes exactly one argument and rejected `packet_loss 10 0`.

Ethernet switch: skip ports whose absorbed anchor does not exist yet (the
join is deferred by design) and roll the ports_mapping back when the VLAN
reconcile fails, so a retry still diffs against the old mapping.

Agent: reject device config entries without a device_name instead of
silently dropping them (a malformed batch used to execute partially while
reporting success).

Also split the netem builder to clear C901 and drop the two per-file ruff
ignores the PR had added, keeping the remove-only policy.

docs/features/docker-kernel-datapath.md

@@ -243,7 +243,7 @@ The veth pairs already exist at attach time — every adapter is born as a veth
 
 ## Capture
 
-Kernel links capture via uBridge's AF_PACKET module bound to the veth host end: `capture start_kernel <if> "<pcap>" [dlt]` / `capture stop_kernel`. Single capture per uBridge process (second concurrent → EALREADY). Start/stop key on the **NIO type**, not on veth presence — a relay NIO riding a veth captures at its relay bridge.
+Kernel links capture via uBridge's AF_PACKET module bound to the veth host end: `capture start_kernel <if> "<pcap>" [dlt]` / `capture stop_kernel`. Single capture per uBridge process (second concurrent → EALREADY): the server tracks which anchor owns the slot, refuses a second port's capture with a clear error instead of leaving it wedged, and only the owning port's stop issues the process-wide `capture stop_kernel`. Start/stop key on the **NIO type**, not on veth presence — a relay NIO riding a veth captures at its relay bridge.
 
 ## Markers