Add follow mode for growing pcap files

[keilogic]

Summary

• add explicit follow_file support to PacketStreamer so pcap files can be polled for appended packets after EOF
• reopen followed pcap files and skip packets already yielded, while preserving the default one-shot pcap-file behavior
• expose --follow / --follow-timeout in the simple packet print example and document the live-updating pcap usage
• add a fake-pcap unit test covering appended-packet behavior without requiring libpcap locally

Fixes #12.

Verification

• py -m py_compile chains\sources\packet_streamer.py chains\sources\packet_streamer_follow_file_test.py examples\simple_packet_print.py
• py -m flake8 chains\sources\packet_streamer.py chains\sources\packet_streamer_follow_file_test.py examples\simple_packet_print.py
• py -m pytest chains\sources\packet_streamer_follow_file_test.py -q
• git diff --check

Note: full local collection is still limited on my Windows/Python 3.13 environment because the current base branch imports pcapy directly and pcapy is not installed there. The new follow-mode test injects a fake pcapy module so the follow/reopen behavior is covered without native libpcap dependencies.