Restore restructured setup.sh (main was broken by 'corrections' merge)

The corrections commit replaced the lib/+modules/ entrypoint with the
older single-file version, reintroducing the DEBUG_MODE unbound-variable
crash, the unguarded clear, and TWM dispatch pointing at nonexistent
root-level scripts. lib/ and modules/ were orphaned.

Restores the tested restructured setup.sh + the SC2034 fix in lib/common.sh.

lib/common.sh

@@ -27,6 +27,8 @@ BOLD='\033[1m'
 RESET='\033[0m'
 
 # Disable colors if output is not a terminal
+# (PURPLE/BLUE/BOLD defined for future use; shellcheck has no way to know that)
+# shellcheck disable=SC2034
 if [ ! -t 1 ]; then
     RED='' GREEN='' YELLOW='' CYAN='' PURPLE='' BLUE='' GRAY='' BOLD='' RESET=''
 fi