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/holder.css

@@ -31,6 +31,28 @@
   letter-spacing: 0.05em;
   margin: 0 0 6px;
   font-weight: 600;
+  cursor: pointer;
+  user-select: none;
+  display: flex;
+  align-items: center;
+  gap: 6px;
+}
+.param-group-title::before {
+  content: "\25BC";   /* ▼ */
+  display: inline-block;
+  font-size: 8px;
+  transition: transform 0.12s ease;
+}
+.param-group.collapsed .param-group-title::before {
+  transform: rotate(-90deg);   /* ▶ */
+}
+.param-group.collapsed .param-group-body {
+  display: none;
+}
+.param-group-body {
+  display: flex;
+  flex-direction: column;
+  gap: 4px;
 }
 
 .param-row {