[WIP] Add PcapFileReader class for reading .pcap files

[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>PCap: file reader</issue_title>
<issue_description>## Problem

Magma.PCap is write-only (PcapFileWriter.cs only). The README describes "Packet capture support for network monitoring" which strongly implies the ability to read captures, not just write them.

Sub-tasks

• PcapFileReader class: open .pcap, validate magic number (both endiannesses), read global header
• Streaming packet enumeration via IEnumerable<PcapPacket> where PcapPacket wraps timestamp + ReadOnlyMemory<byte>
• Link-layer type awareness (Ethernet, raw IP, etc.) from global header
• Optional predicate-based filtering during read

Dependencies

Depends on: #99 (CI)
Unlocks: #112 (PCap round-trip tests)

Phase

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

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

• Fixes PCap: file reader #111

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