Implement proxy chain connection accumulation fix and add comprehensive tests

- Updated socket handling to prevent connection accumulation in chained proxies.
- Introduced centralized bidirectional forwarding for consistent socket management.
- Enhanced cleanup logic to ensure immediate closure of sockets when one closes.
- Added tests to verify connection behavior under various scenarios, including backend failures and rapid reconnections.

readme.hints.md

@@ -582,4 +582,62 @@ onError: (error) => {
 - **Positive**: Prevents resource exhaustion from rapid reconnection attempts
 
 ### Migration Notes
-No configuration changes needed. The fix is automatic and backward compatible.
+No configuration changes needed. The fix is automatic and backward compatible.
+
+## Proxy Chain Connection Accumulation Fix (v19.5.14+)
+
+### Issue
+When chaining SmartProxies (Client → SmartProxy1 → SmartProxy2 → Backend), connections would accumulate and never be cleaned up. This was particularly severe when the backend was down or closing connections immediately.
+
+### Root Cause
+The half-open connection support was preventing proper cascade cleanup in proxy chains:
+1. Backend closes → SmartProxy2's server socket closes
+2. SmartProxy2 keeps client socket open (half-open support)
+3. SmartProxy1 never gets notified that downstream is closed
+4. Connections accumulate at each proxy in the chain
+
+The issue was in `createIndependentSocketHandlers()` which waited for BOTH sockets to close before cleanup.
+
+### Solution
+1. **Changed default behavior**: When one socket closes, both close immediately
+2. **Made half-open support opt-in**: Only enabled when explicitly requested
+3. **Centralized socket handling**: Created `setupBidirectionalForwarding()` for consistent behavior
+4. **Applied everywhere**: Updated HttpProxyBridge and route-connection-handler to use centralized handling
+
+### Changes Made
+```typescript
+// socket-utils.ts - Default behavior now closes both sockets
+export function createIndependentSocketHandlers(
+  clientSocket, serverSocket, onBothClosed,
+  options: { enableHalfOpen?: boolean } = {} // Half-open is opt-in
+) {
+  // When server closes, immediately close client (unless half-open enabled)
+  if (!clientClosed && !options.enableHalfOpen) {
+    clientSocket.destroy();
+  }
+}
+
+// New centralized function for consistent socket pairing
+export function setupBidirectionalForwarding(
+  clientSocket, serverSocket,
+  handlers: {
+    onClientData?: (chunk) => void;
+    onServerData?: (chunk) => void;
+    onCleanup: (reason) => void;
+    enableHalfOpen?: boolean; // Default: false
+  }
+)
+```
+
+### Test Coverage
+- `test/test.proxy-chain-simple.node.ts` - Verifies proxy chains don't accumulate connections
+- Tests confirm connections stay at 0 even with backend closing immediately
+- Works for any proxy chain configuration (not just localhost)
+
+### Performance Impact
+- **Positive**: No more connection accumulation in proxy chains
+- **Positive**: Immediate cleanup reduces memory usage
+- **Neutral**: Half-open connections still available when needed (opt-in)
+
+### Migration Notes
+No configuration changes needed. The fix applies to all proxy chains automatically.