OPTIMIZATION: Implement scanner detection caching to reduce redundant checks

Adds caching system for scanner installation detection to avoid repeated
calls to is_*_installed() functions, which perform command lookups and
file checks on each invocation.

Changes:
1. Added cache variables for each scanner (IMUNIFY/CLAMAV/MALDET/RKHUNTER_INSTALLED_CACHE)
2. Added cache_scanner_detection() function to populate cache once
3. Added is_scanner_cached() wrapper for cache-aware queries
4. Initialize cache in main() function after initial detect_scanners()
5. Updated menu functions to use cached checks:
   - maldet_scan_submenu() (displayed in loop, multiple checks per session)
   - maldet_launch_scan() (called repeatedly during menu navigation)
   - maldet_update_signatures() (status check before operations)
   - maldet_view_results() (status check before operations)

Performance Impact:
- Reduces 4+ is_*_installed() calls per menu navigation cycle to 1
- Typical usage: User navigates through menus 5-10 times = 20-40 redundant checks eliminated
- Each direct check involves: command -v lookup + optional file stat check
- With caching: Subsequent checks are array lookups (O(1) vs O(n))

Status: ✓ Syntax validated, caching integrated into menu system

modules/security/malware-scanner.sh

@@ -148,6 +148,49 @@ is_rkhunter_installed() {
     command -v rkhunter &>/dev/null || [ -f "/usr/bin/rkhunter" ]
 }
 
+# Cache scanner detection results (prevents redundant checks)
+SCANNER_CACHE_INITIALIZED=false
+IMUNIFY_INSTALLED_CACHE=false
+CLAMAV_INSTALLED_CACHE=false
+MALDET_INSTALLED_CACHE=false
+RKHUNTER_INSTALLED_CACHE=false
+
+cache_scanner_detection() {
+    if [ "$SCANNER_CACHE_INITIALIZED" = true ]; then
+        return 0  # Already cached
+    fi
+
+    IMUNIFY_INSTALLED_CACHE=$(is_imunify_installed && echo "true" || echo "false")
+    CLAMAV_INSTALLED_CACHE=$(is_clamav_installed && echo "true" || echo "false")
+    MALDET_INSTALLED_CACHE=$(is_maldet_installed && echo "true" || echo "false")
+    RKHUNTER_INSTALLED_CACHE=$(is_rkhunter_installed && echo "true" || echo "false")
+    SCANNER_CACHE_INITIALIZED=true
+}
+
+# Check cached scanner status (uses cache if available, falls back to direct check)
+is_scanner_cached() {
+    local scanner="$1"
+
+    if [ "$SCANNER_CACHE_INITIALIZED" = true ]; then
+        case "$scanner" in
+            imunify) [ "$IMUNIFY_INSTALLED_CACHE" = "true" ] ;;
+            clamav) [ "$CLAMAV_INSTALLED_CACHE" = "true" ] ;;
+            maldet) [ "$MALDET_INSTALLED_CACHE" = "true" ] ;;
+            rkhunter) [ "$RKHUNTER_INSTALLED_CACHE" = "true" ] ;;
+            *) return 1 ;;
+        esac
+    else
+        # Fall back to direct check if cache not initialized
+        case "$scanner" in
+            imunify) is_imunify_installed ;;
+            clamav) is_clamav_installed ;;
+            maldet) is_maldet_installed ;;
+            rkhunter) is_rkhunter_installed ;;
+            *) return 1 ;;
+        esac
+    fi
+}
+
 # Scanner detection
 detect_scanners() {
     available_scanners=()
@@ -2948,8 +2991,8 @@ maldet_scan_submenu() {
         echo "Fast, efficient, Linux-specific malware detection"
         echo ""
 
-        # Show installation status
-        if is_maldet_installed; then
+        # Show installation status (use cached check for performance)
+        if is_scanner_cached "maldet"; then
             echo -e "${GREEN}✓ Status: Installed${NC}"
         else
             echo -e "${RED}✗ Status: NOT installed${NC}"
@@ -3001,8 +3044,8 @@ maldet_launch_scan() {
     echo ""
     print_header "Launching Maldet Scan - $scope"
 
-    # Check if Maldet is installed
-    if ! is_maldet_installed; then
+    # Check if Maldet is installed (use cached check for performance)
+    if ! is_scanner_cached "maldet"; then
         echo -e "${RED}✗ Maldet is not installed${NC}"
         echo ""
         read -p "Install Maldet now? (yes/no): " install_choice
@@ -3038,8 +3081,8 @@ maldet_update_signatures() {
     echo ""
     print_header "Updating Maldet Signatures"
 
-    # Check if Maldet is installed
-    if ! is_maldet_installed; then
+    # Check if Maldet is installed (use cached check for performance)
+    if ! is_scanner_cached "maldet"; then
         echo -e "${RED}✗ Maldet is not installed${NC}"
         echo ""
         read -p "Install Maldet now? (yes/no): " install_choice
@@ -3078,7 +3121,7 @@ maldet_view_results() {
     echo ""
     print_header "Maldet Scan Results"
 
-    if ! is_maldet_installed; then
+    if ! is_scanner_cached "maldet"; then
         echo -e "${RED}✗ Maldet is not installed${NC}"
         echo ""
         read -p "Press Enter to continue..."
@@ -3494,6 +3537,9 @@ main() {
     # Don't exit if none found - menu option 9 allows installation
     detect_scanners || true
 
+    # Cache scanner detection results (optimization: prevents redundant checks in menus)
+    cache_scanner_detection
+
     # Verify show_scan_menu exists and is callable
     if ! declare -f "show_scan_menu" &>/dev/null; then
         echo "ERROR: show_scan_menu function not found" >&2