[Revert] Remove unsigned MCP pushes to master (P2P sockets foundation)

[cschuchardt88]

Summary

Reverts three commits that were pushed directly to master via GitHub MCP (admin policy override), bypassing required PRs and status checks.

Commits reverted (newest → oldest)

1. d3afefa — Add missing using for StreamExtensions and verify message checksum on receive
2. 2f57140 — Fix Message.Serialize payload write to avoid compact size prefix; expose ComputeChecksum
3. 8b6c13b — Add P2P network sockets foundation for peer-to-peer communication using System.Net.Sockets

Result

Restores master content for this work to the state after #31 (f4ca27f).

Why a PR

Uses the normal PR path (no admin bypass). Please merge so history stays clear and branch rules apply.

Notes

• Commits were unsigned and introduced via github-mcp-server user-to-server token as admin.
• Follow-up: tighten branch rules so admins cannot bypass PR requirements; revoke/limit GitHub MCP write scope if not needed.