_choose_capture_side prefers BUILTIN_NODE_TYPES which includes
ethernet_switch/ethernet_hub/NAT etc. — nodes that do not use uBridge
and therefore have no bridge for a marker filter to attach to.
Add _choose_marker_side with a _MARKER_UNSUPPORTED_TYPES blocklist
so marker capture-side selection skips NIO-only switches and picks
the uBridge-backed node (VPCS/QEMU/Docker/etc.) instead.
Verified end-to-end: PC1↔Switch↔PC2 links with arp markers correctly
pick VPCS nodes; MARK signals flow through to WebSocket and pcaps
are written.