busbars: FDM test-print export (extruded STL + STEP)

Adds a way to print a busbar on a 3D printer to physically verify
sizing before laser-cutting nickel/copper. The existing "Extrude
solid + 0.2mm" path stays for thin-strip STEP — slicers can't print
0.2mm sheet — so the FDM path has its own thickness input (default
2mm, min 0.5mm).

Backend:
- busbar_export.py: new to_stl() writer. Forces extrude_flag = True
  (STL is inherently 3D) and bumps thickness up to 2mm if the
  incoming value is <0.5mm. Registers under WRITERS["stl"] so the
  existing /api/export/<fmt> route serves it for free.

UI:
- index.html: new "FDM test print" block under the Params panel
  with its own thickness input and two buttons (STL, STEP). The
  existing 'Extrude solid' checkbox + 0.2mm thickness keep
  driving plain "Export STEP".
- styles.css: .fdm-block / .fdm-row / .fdm-buttons styles
  matching the existing panel typography.
- app.js: _exportFdm(fmt) reuses Exporter.exportFormat with a
  shallow-merged params override ({extrude: true, thickness: fdmT}),
  so the on-the-fly request gets the FDM settings without
  mutating the live params state.

Verified: STL render of a 3-cell strip @ 2mm => 73KB binary STL
(opens cleanly in slicers); STEP @ 2mm => 160KB ISO-10303-21
solid; existing flat STEP path unchanged at 15KB.

static/js/app.js

@@ -157,6 +157,16 @@
   $("btn-export-dxf" ).addEventListener("click", () => Exporter.exportFormat("dxf",  state, params));
   $("btn-export-svg" ).addEventListener("click", () => Exporter.exportFormat("svg",  state, params));
 
+  // ---- FDM test print -----------------------------------------------------
+  // Forces extrusion at the FDM thickness, regardless of the 'Extrude solid'
+  // checkbox above (which keeps owning the thin nickel-strip case).
+  function _exportFdm(fmt) {
+    const t = Math.max(0.5, +$("p-fdm-thickness").value || 2);
+    Exporter.exportFormat(fmt, state, { ...params, extrude: true, thickness: t });
+  }
+  $("btn-export-fdm-stl") .addEventListener("click", () => _exportFdm("stl"));
+  $("btn-export-fdm-step").addEventListener("click", () => _exportFdm("step"));
+
   // ---- viewport init ------------------------------------------------------
   Viewport.init($("viewport"), state, params, {
     onCellClick: (cellId, mods) => {