docs: add virtual kernel architecture rules to CLAUDE.md

All I/O must route through the virtual kernel. Network loopback is
kernel-mediated. VFS is not the host filesystem. New I/O features
must go through the kernel, never bypass to host directly.

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

Author: NathanFlurry

CLAUDE.md

@@ -107,6 +107,16 @@
 - read `docs-internal/arch/overview.md` for the component map (NodeRuntime, RuntimeDriver, NodeDriver, NodeExecutionDriver, ModuleAccessFileSystem, Permissions)
 - keep it up to date when adding, removing, or significantly changing components
 
+## Virtual Kernel Architecture
+
+- **all sandbox I/O routes through the virtual kernel** — user code never touches the host OS directly
+- the kernel provides: VFS (virtual file system), process table (spawn/signals/exit), network stack (TCP/HTTP/DNS/UDP), and a deny-by-default permissions engine
+- **network calls are kernel-mediated**: `http.createServer()` registers a virtual listener in the kernel's network stack; `http.request()` to localhost routes through the kernel without real TCP — the kernel connects virtual server to virtual client directly; external requests go through the host adapter after permission checks
+- **the VFS is not the host file system** — files written by sandbox code live in the VFS (in-memory by default); host filesystem is accessible only through explicit read-only overlays (e.g., `node_modules`) configured by the embedder
+- **embedders provide host adapters** that implement actual I/O — a Node.js embedder provides real `fs` and `net`; a browser embedder provides `fetch`-based networking and no file system; sandbox code doesn't know which adapter backs the kernel
+- when implementing new I/O features (e.g., UDP, TCP servers, fs.watch), they MUST route through the kernel — never bypass it to hit the host directly
+- see `docs/nodejs-compatibility.mdx` for the architecture diagram
+
 ## Code Transformation Policy
 
 - NEVER use regex-based source code transformation for JavaScript/TypeScript (e.g., converting ESM to CJS, rewriting imports, extracting exports)