Changes:
- Replace tshark BPF validation with tcpdump -d (calls pcap_compile
internally like ubridge, returns instantly without waiting for traffic)
- Support multi-line BPF expressions: split on newlines and validate
each line individually
- Always validate, never save invalid filters on error
- Drop invalid filters during project load with warning (prevents
old topologies with bad filters from failing to open)
- Simplify test cases (no longer depend on tshark availability)
7bcb96a368