feat: concurrent kernel captures, one per interface

uBridge's capture module now serves one AF_PACKET capture per interface,
any number per process, and `capture stop_kernel` takes the interface it
stops — this rides that (breaking, paired-binary) protocol change.

The server-side single-slot tracker (_kernel_capture_ifc with its
reserve/release/owned_by helpers) is gone: same-port duplicates were
already refused by the NIO capturing flag, uBridge answers EALREADY on a
same-interface duplicate, and per-interface stops removed the process-
wide stop the ownership gating guarded against.

The docker e2e drives ten concurrent captures on one node's uBridge and
asserts a per-interface stop freezes exactly the stopped half while the
live half keeps recording.

gns3server/compute/base_node.py

@@ -1057,10 +1057,6 @@ class BaseNode:
         # than skipping them as "already installed".
         self._marker_filter_bridges.clear()
         self._marker_specs.clear()
-        # ...and so is the kernel capture, uBridge's single AF_PACKET slot
-        # (meaningful for the kernel-datapath node types; see
-        # KernelDatapathMixin._reserve_kernel_capture).
-        self._kernel_capture_ifc = None
 
     async def add_ubridge_udp_connection(self, bridge_name, source_nio, destination_nio):
         """