cdp: send a going-away close frame (1001) on pending terminate

Previously tick() tore the connection down without a WebSocket close
frame, so clients saw an abnormal closure (1006). Safe to send from
tick(): the worker thread is the sole writer of the socket.

src/cdp/CDP.zig

@@ -246,6 +246,11 @@ pub fn tick(self: *CDP) !bool {
     // so the flag can't be a stale leftover here. Exit.
     if (self.browser.env.terminatePending()) {
         log.warn(.cdp, "closing connection", .{ .reason = "pending terminate" });
+        // The worker thread is the sole writer of this socket, so sending the
+        // close frame here can't interleave with another write.
+        self.conn.send(&WS.CLOSE_GOING_AWAY) catch |err| {
+            log.warn(.app, "CDP terminate close", .{ .err = err });
+        };
         return false;
     }
 
@@ -1435,6 +1440,25 @@ test "cdp: disconnect latches so the worker keeps exiting" {
     try testing.expectError(error.ClientDisconnected, client.tick(0));
 }
 
+test "cdp: tick sends a close frame on pending terminate" {
+    var ctx = try testing.context();
+    defer ctx.deinit();
+
+    const cdp = ctx.cdp();
+    cdp.browser.env.requestTerminate();
+    // Clear the pending terminate so deinit's V8 calls don't trip over the
+    // terminating-state asserts.
+    defer cdp.browser.env.cancelTerminate();
+
+    try testing.expectEqual(false, try cdp.tick());
+
+    // The client should receive a close frame (code 1001, going away), not
+    // just an abrupt socket close.
+    var buf: [WS.CLOSE_GOING_AWAY.len]u8 = undefined;
+    const n = try posix.read(ctx.socket, &buf);
+    try testing.expectEqualSlices(u8, &WS.CLOSE_GOING_AWAY, buf[0..n]);
+}
+
 test "cdp: syncRequest short-circuits after disconnect" {
     var ctx = try testing.context();
     defer ctx.deinit();

src/network/WS.zig

@@ -26,6 +26,7 @@ pub const EMPTY_PONG = [_]u8{ 138, 0 };
 
 // CLOSE, 2 length, code
 pub const CLOSE_NORMAL = [_]u8{ 136, 2, 3, 232 }; // code: 1000
+pub const CLOSE_GOING_AWAY = [_]u8{ 136, 2, 3, 233 }; // code: 1001
 pub const CLOSE_TOO_BIG = [_]u8{ 136, 2, 3, 241 }; // 1009
 pub const CLOSE_PROTOCOL_ERROR = [_]u8{ 136, 2, 3, 234 }; //code: 1002