fix(install): replace declare -A with bash 3 compatible string set

macOS ships bash 3.2 which does not support associative arrays (declare -A,
bash 4+ only). Replace INSTALL_SET associative array with a space-delimited
string and an in_install_set() helper function. All operations preserved:
add, skip/remove, empty-check, iteration, membership test.

install.sh

@@ -88,25 +88,33 @@ EOF
 done
 
 # ── build install set ─────────────────────────────────
-# Populates INSTALL_SET associative array: keys are bare names (no .ts suffix).
-declare -A INSTALL_SET
+# INSTALL_SET: space-delimited bare names (no .ts suffix). Bash 3 compatible.
+INSTALL_SET=""
+
+in_install_set() { [[ " $INSTALL_SET " == *" $1 "* ]]; }
 
 build_install_set() {
+    local n f bare entry new
     if [[ -n "$ONLY" ]]; then
         # Explicit allowlist — only install what's named
         IFS=',' read -ra names <<< "$ONLY"
         for n in "${names[@]}"; do
-            INSTALL_SET["${n%.ts}"]=1
+            INSTALL_SET="${INSTALL_SET:+$INSTALL_SET }${n%.ts}"
         done
     else
         # Start with everything present on disk, then remove --skip entries
         for f in "${EXTENSIONS_SRC}"/*.ts; do
-            [[ -e "$f" ]] && INSTALL_SET["$(basename "$f" .ts)"]=1
+            [[ -e "$f" ]] && INSTALL_SET="${INSTALL_SET:+$INSTALL_SET }$(basename "$f" .ts)"
         done
         if [[ -n "$SKIP" ]]; then
             IFS=',' read -ra names <<< "$SKIP"
             for n in "${names[@]}"; do
-                unset "INSTALL_SET[${n%.ts}]" 2>/dev/null || true
+                bare="${n%.ts}"
+                new=""
+                for entry in $INSTALL_SET; do
+                    [[ "$entry" == "$bare" ]] || new="${new:+$new }$entry"
+                done
+                INSTALL_SET="$new"
             done
         fi
     fi
@@ -122,13 +130,13 @@ do_install() {
     require_pi_installed
     build_install_set
 
-    if [[ ${#INSTALL_SET[@]} -eq 0 ]]; then
+    if [[ -z "$INSTALL_SET" ]]; then
         warn "No extensions selected — nothing to install."
         exit 0
     fi
 
     echo "==> Extensions to symlink into ${EXTENSIONS_DEST}/:"
-    for n in "${!INSTALL_SET[@]}"; do
+    for n in $INSTALL_SET; do
         printf '       %s.ts\n' "$n"
     done
     echo
@@ -140,7 +148,7 @@ do_install() {
         local name
         name="$(basename "$src")"
         local bare="${name%.ts}"
-        [[ -n "${INSTALL_SET[$bare]:-}" ]] || continue
+        in_install_set "$bare" || continue
 
         local dest="${EXTENSIONS_DEST}/${name}"