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>

index.html

@@ -66,6 +66,20 @@
     button.primary { background: #238636; border-color: #2ea043; }
     button.wasm { border-color: var(--wasm); color: var(--wasm); }
     label.btn input { display: none; }
+    /* 선가중치 표시 배율 조절 (슬라이더 + 직접 입력 + 환산 결과) */
+    #lwScaleBox {
+      display: flex; align-items: center; gap: 6px;
+      background: var(--panel); border: 1px solid var(--line);
+      border-radius: 8px; padding: 5px 10px; font: 12px var(--mono); color: var(--muted);
+    }
+    #lwScaleBox input[type=range] { width: 110px; accent-color: var(--accent); }
+    #lwScaleBox input[type=number] {
+      width: 52px; background: #0d1117; color: var(--ink);
+      border: 1px solid var(--line); border-radius: 6px; padding: 3px 5px;
+      font: 12px var(--mono); text-align: right;
+    }
+    #lwScaleBox .unit { color: var(--muted); }
+    #lwScaleOut { color: var(--accent); white-space: nowrap; }
     #status {
       margin-left: auto; font: 12px var(--mono); color: var(--muted);
       background: rgba(22,27,34,.85); border: 1px solid var(--line);
@@ -214,6 +228,14 @@
     <button id="fit" type="button">Fit (F)</button>
     <button id="theme" type="button">Theme</button>
     <button id="lwt" type="button" title="선가중치 표시 (AutoCAD LWDISPLAY)">선가중치</button>
+    <!-- 표시 배율: AutoCAD "선가중치 설정 → 표시 배율" 슬라이더에 해당.
+         리로드 없이 즉시 반영된다. -->
+    <div id="lwScaleBox" title="선가중치 표시 배율 (px / mm)">
+      <input id="lwScale" type="range" min="2" max="60" step="1" value="20" />
+      <input id="lwScaleNum" type="number" min="1" max="80" step="1" value="20" />
+      <span class="unit">px/mm</span>
+      <span id="lwScaleOut">—</span>
+    </div>
     <button id="layersBtn" type="button">Layers</button>
     <button id="propsBtn" type="button">Props (속성)</button>
     <div id="status">ready</div>