Fix buffer capacity check in varint encode to account for byteOffset (#127)

The capacity check was comparing against the buffer's total byteLength
without accounting for byteOffset. For Uint8Array views over a larger
buffer, this could incorrectly allow writes that exceed the available space.

Use dst.byteOffset + dst.byteLength + size > dst.buffer.byteLength to
properly check the actual end position of the appended data.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[email protected]>

Author: kixelated

web-transport-ws/src/varint.ts

@@ -28,7 +28,7 @@ export class VarInt {
 		const x = this.value;
 
 		const size = this.size();
-		if (dst.buffer.byteLength < dst.byteLength + size) {
+		if (dst.byteOffset + dst.byteLength + size > dst.buffer.byteLength) {
 			throw new Error("destination buffer too small");
 		}