perf(purge/uninstall): eliminate per-item subprocess forks in scan phase

purge:
- is_safe_project_artifact: replace echo|tr|wc depth calc with pure bash
  string arithmetic (2 forks → 0 per item)
- process_scan_results: replace dirname subprocess with ${item%/*}
- is_recently_modified: accept pre-computed epoch to avoid redundant
  get_epoch_seconds call (N date forks → 1 before loop)
- fd pattern construction: replace 45 per-target sed forks with single
  printf|sed pass (45 forks → 2)
- pre-compute _cached_project_paths alongside existing name/basename
  cache so display loop avoids get_project_path subshell per item

uninstall/app-selector:
- default size display: N/A → -- for apps without cached size data
  (cleaner UX on first scan; real sizes populate from background refresh)
- Pass 1 scan: replace basename/dirname subshells with parameter expansion

bin/uninstall.sh

@@ -469,12 +469,11 @@ scan_applications() {
         while IFS= read -r -d '' app_path; do
             if [[ ! -e "$app_path" ]]; then continue; fi
 
-            local app_name
-            app_name=$(basename "$app_path" .app)
+            local app_name="${app_path##*/}"
+            app_name="${app_name%.app}"
 
             # Skip nested apps inside another .app bundle.
-            local parent_dir
-            parent_dir=$(dirname "$app_path")
+            local parent_dir="${app_path%/*}"
             if [[ "$parent_dir" == *".app" || "$parent_dir" == *".app/"* ]]; then
                 continue
             fi
@@ -485,9 +484,8 @@ scan_applications() {
                 if [[ -n "$link_target" ]]; then
                     local resolved_target="$link_target"
                     if [[ "$link_target" != /* ]]; then
-                        local link_dir
-                        link_dir=$(dirname "$app_path")
-                        resolved_target=$(cd "$link_dir" 2> /dev/null && cd "$(dirname "$link_target")" 2> /dev/null && pwd)/$(basename "$link_target") 2> /dev/null || echo ""
+                        local link_dir="${app_path%/*}"
+                        resolved_target=$(cd "$link_dir" 2> /dev/null && cd "${link_target%/*}" 2> /dev/null && pwd)/"${link_target##*/}" 2> /dev/null || echo ""
                     fi
                     case "$resolved_target" in
                         /System/* | /usr/bin/* | /usr/lib/* | /bin/* | /sbin/* | /private/etc/*)
@@ -661,7 +659,7 @@ scan_applications() {
         fi
 
         local final_size_kb=0
-        local final_size="N/A"
+        local final_size="--"
         if [[ "$cached_size_kb" =~ ^[0-9]+$ && $cached_size_kb -gt 0 ]]; then
             final_size_kb="$cached_size_kb"
             final_size=$(bytes_to_human "$((cached_size_kb * 1024))")