holder: expose all 59 scad parameters auto-extracted from the .scad source

The bundled hex_cell.scad has many more knobs than the dozen we used to
expose (box clearances, cap/box wall, vertical stacking, busbar template,
etc). Maintaining a hand-curated PARAMS list in Python next to a SCAD file
that already documents every variable inline was always going to drift.

Sync + auto-extract approach:
- scad/hex_cell.scad: replace with the upstream master file from
  Albert Phan's Hex-Cell-Holder fork (adds the BUSBAR TEMPLATE section
  and the "busbar template" part option).
- holder.py: PARAMS now built at import time by _scan_scad(), which
  regexes top-level `name = literal;  // help` lines into Param entries
  up until the // END OF CONFIGURATION marker. Scan handles bool /
  number / string literals; derived expressions and helper variables
  are skipped automatically.
- Manual maps stay small and explicit: _SELECT_OPTIONS for the few
  string-enum params (part, pack_style, box_style, template_outline,
  template_hole_style, etc.), _GROUP_RULES for UI sectioning, and
  _NUMBER_HINTS for sensible min/max/step on the most-tweaked numbers.

UI:
- holder-app.js: extra GROUP_ORDER / GROUP_LABELS for the new
  sections (cap, box, insulator, bolts, wires, stacking, template,
  advanced). Group titles are now click-to-collapse; the advanced
  groups start collapsed so the form isn't a wall of inputs on load.
- holder.css: caret marker on the group title, smooth rotate on
  collapse, hides the body via .collapsed class.

Net effect: every variable in the .scad — including the new busbar
template knobs — is editable from the page, with helpful comments
copied straight from the .scad source.

static/js/holder-app.js

@@ -53,13 +53,31 @@ function _whenViewerReady(cb) {
 
 // ----- form generation ------------------------------------------------------
 
-const GROUP_ORDER = ["part", "cell", "size", "holder"];
+const GROUP_ORDER = [
+  "part", "cell", "size", "holder",
+  "cap", "box", "insulator",
+  "bolts", "wires",
+  "stacking", "template", "advanced",
+];
 const GROUP_LABELS = {
-  part:   "Part / pack",
-  cell:   "Cell",
-  size:   "Pack size",
-  holder: "Holder",
+  part:      "Part / pack",
+  cell:      "Cell",
+  size:      "Pack size",
+  holder:    "Holder",
+  cap:       "Cap",
+  box:       "Box",
+  insulator: "Insulator",
+  bolts:     "Bolts & zipties",
+  wires:     "Wires & clamp",
+  stacking:  "Vertical stacking",
+  template:  "Busbar template",
+  advanced:  "Advanced",
 };
+// Groups that start collapsed (user opens with a click). Basics stay open.
+const GROUPS_COLLAPSED = new Set([
+  "cap", "box", "insulator", "bolts", "wires",
+  "stacking", "template", "advanced",
+]);
 
 function _renderForm(schema, defaults) {
   const root = $("param-form");
@@ -77,11 +95,18 @@ function _renderForm(schema, defaults) {
   for (const g of groups) {
     const wrap = document.createElement("div");
     wrap.className = "param-group";
+    if (GROUPS_COLLAPSED.has(g)) wrap.classList.add("collapsed");
+
     const title = document.createElement("h3");
     title.className = "param-group-title";
     title.textContent = GROUP_LABELS[g] || g;
+    title.addEventListener("click", () => wrap.classList.toggle("collapsed"));
     wrap.appendChild(title);
 
+    const body = document.createElement("div");
+    body.className = "param-group-body";
+    wrap.appendChild(body);
+
     for (const p of byGroup.get(g)) {
       const row = document.createElement("div");
       row.className = "param-row";
@@ -123,7 +148,7 @@ function _renderForm(schema, defaults) {
         h.textContent = p.help;
         row.appendChild(h);
       }
-      wrap.appendChild(row);
+      body.appendChild(row);
     }
     root.appendChild(wrap);
   }