fix: install-extras' preselect loop never ran more than once

for each in {1..${#_extras[@]}} doesn't expand as a range - bash does
brace expansion before parameter expansion, so the range endpoint
isn't resolved yet when brace expansion runs. The loop always ran
exactly once, leaving preselect at length 1 regardless of how many
extras exist. Invisible until now because there was only ever one
extra (model-cache-redirect); would have silently broken any
preselected-true default on a 2nd+ extra. Switched to a C-style loop.

This is almost certainly the multiselect misbehavior from years back -
tracked down while verifying multiselect() itself (functions/PRbL
submodule, fixed separately) didn't have a toggle-isolation bug.

install.sh

@@ -318,7 +318,13 @@ install-extras(){
     # done
 
     boxborder "Which extras should be installed?"
-    for each in {1..${#_extras[@]}} ; do
+    # NOTE: {1..${#_extras[@]}} does NOT expand as a range - bash performs
+    # brace expansion before parameter expansion, so the range's endpoint
+    # isn't resolved yet when brace expansion runs. This always looped
+    # exactly once (leaving preselect at length 1 regardless of extras
+    # count), invisible only because there was ever just one extra. Use a
+    # C-style loop instead, which resolves ${#_extras[@]} correctly.
+    for ((each=0; each<${#_extras[@]}; each++)); do
         preselect+=("false")
     done
     multiselect result _extras preselect