`end()` uses non-null assertion `this.ws!.close()` but `ws` is `null` until async assignment — TypeError on early call

Bug

In src/shims/net/index.ts around line 725, the end() method closes the WebSocket using a non-null assertion:

this.ws!.close();

However, this.ws is initialized to null and is only set asynchronously (in the Cloudflare fetch path, after the WebSocket upgrade completes). If end() is called before ws has been assigned — for example, when called with an empty buffer where the write callback fires immediately — this.ws is still null at that point and the ! operator produces a TypeError: Cannot read properties of null (reading 'close').

Issue #131 fixed the same class of null assertion in rawWrite() but end() was missed.

Reproduction

const socket = new NeonSocket(...);
socket.end(); // ws not yet assigned — TypeError: Cannot read properties of null

This also surfaces when end() is called during connection teardown before the WebSocket upgrade has resolved.

Fix

Guard against null before calling .close():

// Before
this.ws!.close();

// After
if (this.ws) {
  this.ws.close();
}

This is consistent with the fix applied in #131 to rawWrite().

Start in src/shims/net/index.ts around line 725 and compare end() with the null-handling fix in rawWrite() from issue #131. Verify the early end() reproduction and confirm that teardown before the WebSocket upgrade no longer raises a null close TypeError.