Add comprehensive documentation for delivery confirmation feature

- Complete implementation guide
- Test checklist with expected logs
- Explanation of how ephemeral peer IDs are handled
- Visual indicator reference

DELIVERY_TEST_CHECKLIST.md

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+# Test Checklist for Delivery Confirmation
+
+## Quick Test
+1. **Rick sends to Jack** (while Jack is not in the chat)
+   - ✓ Rick sees gray circle → gray check → green double checks
+   
+2. **Jack opens chat with Rick**
+   - ✓ Jack should see Rick's message
+   - ✓ Rick should see blue double checks (this is what we fixed!)
+
+3. **Jack sends reply to Rick** (while Rick is viewing)
+   - ✓ Jack sees gray circle → gray check → green double checks → blue double checks
+
+## Debug Logs to Watch For
+
+**On Jack's device when opening chat:**
+```
+[UI] Selected private chat peer changed to [peer-id]
+[UI] Triggering markPrivateMessagesAsRead for peer [peer-id]
+[Delivery] Checking N messages in chat with peer [peer-id] (rick) for read receipts
+[Delivery] Message [id] from rick, senderPeerID: [old-id], currentPeerID: [new-id], myNickname: jack, status: Delivered to jack
+[Delivery] Sending read receipt for message [id] from rick to current peer [peer-id]
+[DeliveryTracker] Broadcasting read receipt packet to [peer-id]
+```
+
+**On Rick's device when receiving read receipt:**
+```
+[Delivery] Received READ receipt for message [id] from jack
+[Delivery] Updating message [id] to status: read(by: "jack", at: [timestamp])
+[UI] Showing BLUE checkmarks for read status by jack
+```
+
+## What to Check
+- Messages sent before opening chat should turn blue when chat is opened
+- Peer ID changes between sessions shouldn't break read receipts
+- No race conditions - blue checkmarks should stay blue
+- Read receipts work for both real-time and delayed chat opening