feat(viewer2d): make the lineweight display scale adjustable from the toolbar

The px-per-mm mapping was a hard-coded guess twice over, and the right
value depends on the monitor - AutoCAD has the same knob as the Adjust
Display Scale slider. Expose it: a slider plus a number box next to the
lineweight toggle, with a readout of what the drawing's own weights come
out to (0.35mm -> 7px, 0.50mm -> 10px at the 20 px/mm default). The
value persists in localStorage.

Retuning applies to the live materials instead of rebuilding the scene.
The hairline cut-off is a millimetre value, so changing the scale moves
every weight by the same factor without changing which bucket a segment
belongs to - each fat batch now remembers its weight in mm, and only
material.linewidth is rewritten. That keeps a drag on the slider instant
even on the 37 MB road drawing.

Measured headless on its 0.35mm gutter line: 2-3px at scale 8, 7px at
20, 14px at 40 - matching the readout in each case.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/main.ts

@@ -163,6 +163,7 @@ async function loadAndRender(buf: ArrayBuffer, name: string): Promise<CadParseRe
   viewer.load(result, { spaceHandle: def?.handleHex ?? null });
   viewer.resize();
   syncLwtBtn();
+  syncLwScale();
   return result;
 }
 
@@ -240,7 +241,39 @@ const syncLwtBtn = () => {
 lwtBtn.addEventListener('click', () => {
   viewer.setLineweightEnabled(!viewer.getLineweightEnabled());
   syncLwtBtn();
+  syncLwScale();
 });
+
+// 선가중치 표시 배율 (px per mm) — AutoCAD "표시 배율" 슬라이더에 해당.
+// 리로드 없이 즉시 반영되고, 값은 localStorage 에 남는다.
+const LW_SCALE_KEY = 'hmw:lw-scale';
+const lwScaleRange = document.getElementById('lwScale') as HTMLInputElement;
+const lwScaleNum = document.getElementById('lwScaleNum') as HTMLInputElement;
+const lwScaleOut = document.getElementById('lwScaleOut') as HTMLSpanElement;
+
+/** 현재 배율에서 도면이 실제로 쓰는 굵기들이 몇 px 로 그려지는지 표기. */
+function syncLwScale() {
+  const s = viewer.getLineweightScale();
+  lwScaleRange.value = String(s);
+  lwScaleNum.value = String(s);
+  const classes = viewer.getLineweightClasses();
+  lwScaleOut.textContent = classes.length
+    ? classes.map((mm) => `${mm.toFixed(2)}mm→${(Math.round(mm * s * 10) / 10)}px`).join('  ')
+    : `0.50mm→${Math.round(0.5 * s * 10) / 10}px`;
+}
+
+function applyLwScale(v: number) {
+  if (!Number.isFinite(v) || v <= 0) return;
+  viewer.setLineweightScale(v);
+  localStorage.setItem(LW_SCALE_KEY, String(v));
+  syncLwScale();
+}
+lwScaleRange.addEventListener('input', () => applyLwScale(parseFloat(lwScaleRange.value)));
+lwScaleNum.addEventListener('input', () => applyLwScale(parseFloat(lwScaleNum.value)));
+
+const savedLwScale = parseFloat(localStorage.getItem(LW_SCALE_KEY) || '');
+if (Number.isFinite(savedLwScale) && savedLwScale > 0) viewer.setLineweightScale(savedLwScale);
+syncLwScale();
 document.getElementById('layersBtn')!.addEventListener('click', () => {
   layersPanel.style.display = layersPanel.style.display === 'flex' ? 'none' : 'flex';
 });