docs: add kernel networking consolidation spec

Covers virtual socket table, loopback routing, server sockets, UDP,
Unix domain sockets, socket options, network permissions, signal
handlers, VFS change notifications, and migration of 13 Node.js
bridge components to the shared kernel. Includes standalone test
strategy and migration order.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: NathanFlurry