docs: propose a hermetic Node.js runtime

[longlho]

Summary

This adds an RFC for an opt-in hermetic Node.js runtime driven by an immutable
execution manifest and a process-wide virtual host.

The proposal keeps Node independent of any build system, gives node:fs, CJS,
ESM, workers, and loaders one logical filesystem, and lays out focused patches
for startup policy, filesystem enforcement, module identity, ambient inputs,
and a strict runtime target.

It also keeps the security boundary explicit: Node can govern Node-owned
access, while native addons, FFI, and arbitrary child executables still require
OS isolation.

Validation

Documentation-only change. git diff --check passes, and the RFC is wrapped to
the repository's existing Markdown style.

[longlho]

@dzbarsky 1st pass :)