Fix missing shutdown validation in start_second_instance()

- Apply proper shutdown validation to pre-startup cleanup (line 881-899)
  If a stale socket exists, wait for it to be removed instead of just
  sleeping 2 seconds. Uses same pattern as stop_second_instance().

- Apply proper shutdown validation to error path (line 937-960)
  When InnoDB errors are detected, use validated shutdown with socket
  removal verification instead of fire-and-forget mysqladmin call.

- All 4 shutdown paths now consistently:
  1. Send graceful shutdown
  2. Wait for socket file to disappear
  3. Clean up stale socket/lock files
  4. Verify process termination

This ensures no stale processes/sockets remain that could cause crashes
on subsequent script runs.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

modules/backup/mysql-restore-to-sql.sh

@@ -878,8 +878,22 @@ start_second_instance() {
     # Check if socket already exists (instance already running)
     if [ -S "$datadir/socket.mysql" ]; then
         print_warning "Socket file already exists. Attempting to shut down existing instance..."
+
+        # Use proper shutdown validation (same as stop_second_instance)
         mysqladmin -h localhost -S "$datadir/socket.mysql" shutdown 2>/dev/null || true
-        sleep 2
+
+        # Wait for socket to disappear (up to 5 seconds)
+        local cleanup_wait=0
+        while [ -S "$datadir/socket.mysql" ] && [ "$cleanup_wait" -lt 5 ]; do
+            sleep 1
+            cleanup_wait=$((cleanup_wait + 1))
+        done
+
+        # If socket still exists, try force removal
+        if [ -S "$datadir/socket.mysql" ]; then
+            print_warning "Existing instance didn't shut down cleanly. Force removing socket..."
+            rm -f "$datadir/socket.mysql" "$datadir/mysql.lock" 2>/dev/null || true
+        fi
     fi
 
     # Build mysqld command
@@ -924,7 +938,22 @@ start_second_instance() {
                 print_error "InnoDB initialization encountered errors"
                 echo ""
                 print_warning "Attempting to shut down second instance..."
+
+                # Use proper shutdown validation instead of fire-and-forget
                 mysqladmin -h localhost -S "$datadir/socket.mysql" shutdown 2>/dev/null || true
+
+                # Wait for socket to disappear (up to 5 seconds)
+                local error_cleanup_wait=0
+                while [ -S "$datadir/socket.mysql" ] && [ "$error_cleanup_wait" -lt 5 ]; do
+                    sleep 1
+                    error_cleanup_wait=$((error_cleanup_wait + 1))
+                done
+
+                # Remove stale socket/lock if still present
+                if [ -S "$datadir/socket.mysql" ]; then
+                    rm -f "$datadir/socket.mysql" "$datadir/mysql.lock" 2>/dev/null || true
+                fi
+
                 echo ""
                 print_info "Review full error log:"
                 echo "  tail -100 $datadir/mysql.err"