Read the serial line high when no link peer is connected

Peer2PeerSerialEndpoint.sendBit() returned 0 when no peer was attached,
so an internal-clock serial transfer with no cable filled SB with 0x00.
An unconnected link cable actually floats high, so the null endpoint
(and real hardware) return 0xFF. Single-player sessions always wire a
Peer2PeerSerialEndpoint for potential netplay, so every link-aware game
saw a phantom partner: Alleyway runs a serial handshake at stage start,
misdetected a connected Game Boy, and hung waiting for it with the
paddle frozen - the reported input failure (issue #63). sendBit() now
returns 1 when peer is null, matching the null endpoint; the connected
path is unchanged so netplay is unaffected.

Full battery and the controller/netplay suites stay green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01J1wLWscyUGS7CFwc3CjJR8

Author: trekawek

core/src/main/java/eu/rekawek/coffeegb/core/serial/Peer2PeerSerialEndpoint.java

@@ -51,7 +51,11 @@ public void startSending() {
     @Override
     public int sendBit() {
         if (peer == null) {
-            return 0;
+            // an unconnected link cable reads high, so an internal-clock transfer with no
+            // peer receives all ones (SB becomes 0xFF), like the null endpoint. Returning
+            // 0 here made link-aware games see a phantom partner and hang waiting for it
+            // (Alleyway freezes with the paddle stuck, issue #63).
+            return 1;
         }
         peer.bitsReceived.incrementAndGet();
         return shift();