[WIP] Add round-trip tests for PCap writer and reader

[Copilot]

Thanks for assigning this issue to me. I'm starting to work on it and will keep this PR's description up to date as I form a plan and make progress.

Original prompt

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This section details on the original issue you should resolve

<issue_title>test: PCap round-trip</issue_title>
<issue_description>## Problem

PCap needs tests covering both the existing writer and the new reader.

Sub-tasks

• Create test/Magma.PCap.Facts project, add to solution
• Write N packets via PcapFileWriter, read back via PcapFileReader, assert equality
• Test both endianness variants
• Include sample .pcap fixture files from real captures (Wireshark samples)
• Verify link-layer type propagation

Dependencies

Depends on: #111
Unlocks: #123 (pcap-ng + live capture), also useful for testing other protocol layers against real captures

Phase

Phase 1 (Core Protocol & Tooling)</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes test: PCap round-trip #112

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