ifcviewer: marquee box-select on web + suppress the canvas context menu

Bring rubber-band box-select to the web on the Web preset's select button (RMB).

- Core: factor the pick-pass encode + rect copy out of picksInRect into
  encodeBoxPickToStaging (mirroring how single-pick shares
  encodePickReadbackToStaging), shared by the sync picksInRect (desktop) and a
  new async picksInRectAsync (web) — the latter maps the staging buffer via a
  spontaneous callback because the sync spin-map hangs the JS loop. New
  applyMarqueeToSelection (plain replace / Shift add / Ctrl remove).
- Web main_web: a select-button drag past the click threshold draws a marquee
  rubber-band (a plain DOM <div> positioned in CSS px — no GPU overlay pass,
  which the web lib lacks) and on release box-picks the rect (device px) and
  applies it to the selection. A click (no drag) still single-picks.
- Web shell.html: the #marquee div + styling, and — the reported bug — a
  contextmenu preventDefault on the canvas so RMB (now the select button) doesn't
  pop the browser menu. (Firefox still forces its native menu on Shift+RightClick;
  that's a browser escape hatch pages can't override.)

Tests: applyMarqueeToSelection replace/add/remove + id-0 (Catch2, 124 total);
web smoke marquee drag → rubber-band shown → selection changes → hidden (10/10).
Desktop picksInRect unchanged in behaviour; BonsaiViewer builds.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ifcviewer-web/tests/smoke.spec.mjs

@@ -343,3 +343,32 @@ test('hide selected removes geometry after a pick', async ({ page }) => {
   await page.waitForTimeout(400);
   expect(gpuErrors, gpuErrors.join('\n')).toEqual([]);
 });
+
+test('RMB marquee drag box-selects (Web preset)', async ({ page }) => {
+  const gpuErrors = [];
+  page.on('console', (m) => { if (/Uncaptured WebGPU error|is invalid/i.test(m.text())) gpuErrors.push(m.text()); });
+  await ready(page);
+
+  const box = await page.locator('#viewer-canvas').boundingBox();
+  const cx = box.x + box.width / 2, cy = box.y + box.height / 2;
+  const before = await shot(page);
+  await page.mouse.move(cx - 120, cy - 90);
+  await page.mouse.down({ button: 'right' });
+  await page.mouse.move(cx - 40, cy - 30, { steps: 4 });
+  const midDragVisible = await page.evaluate(() => {
+    const m = document.getElementById('marquee');
+    return !!(m && getComputedStyle(m).display !== 'none');
+  });
+  await page.mouse.move(cx + 120, cy + 90, { steps: 6 });
+  await page.mouse.up({ button: 'right' });
+  await page.waitForTimeout(600);  // async box-pick + apply + render
+  const after = await shot(page);
+  const hiddenAfter = await page.evaluate(() => {
+    const m = document.getElementById('marquee');
+    return !!(m && getComputedStyle(m).display === 'none');
+  });
+  expect(midDragVisible, 'marquee rubber-band was not shown during the drag').toBe(true);
+  expect(Buffer.compare(before, after), 'box-select did not change the canvas').not.toBe(0);
+  expect(hiddenAfter, 'marquee was not hidden after release').toBe(true);
+  expect(gpuErrors, gpuErrors.join('\n')).toEqual([]);
+});