refactor: Use XDG compliant isolated directory structure

- Using $BOOTSTRAP_BIN, $BOOTSTRAP_OPT, etc
- Add defensive fallback for undefined vars in common.sh

lib/common.sh

@@ -7,6 +7,15 @@ if [ -n "${_LIB_COMMON_SOURCED:-}" ]; then
 fi
 _LIB_COMMON_SOURCED=1
 
+# Export global environment paths with default fallbacks
+export BOOTSTRAP_DIR="${BOOTSTRAP_DIR:-$HOME/.config/bootstrap}"
+export BOOTSTRAP_DATA_DIR="${BOOTSTRAP_DATA_DIR:-$HOME/.local/share/bootstrap}"
+export BOOTSTRAP_STATE_DIR="${BOOTSTRAP_STATE_DIR:-$HOME/.local/state/bootstrap}"
+export BOOTSTRAP_CACHE_DIR="${BOOTSTRAP_CACHE_DIR:-$HOME/.cache/bootstrap}"
+export BOOTSTRAP_BIN="${BOOTSTRAP_BIN:-$BOOTSTRAP_DATA_DIR/bin}"
+export BOOTSTRAP_OPT="${BOOTSTRAP_OPT:-$BOOTSTRAP_DATA_DIR/opt}"
+export BOOTSTRAP_RUNTIMES="${BOOTSTRAP_RUNTIMES:-$BOOTSTRAP_DATA_DIR/runtimes}"
+
 # Ensure running in Bash
 require_bash() {
     if [ -z "${BASH_VERSION:-}" ]; then
@@ -88,7 +97,7 @@ version_lt() {
 download_file() {
     local url="$1"
     local dest="$2"
-    local cache_dir="$HOME/.local/state/bootstrap/cache"
+    local cache_dir="$BOOTSTRAP_CACHE_DIR/downloads"
     
     mkdir -p "$cache_dir"