Draw a silhouette outline around the selection

The renderer tints the selection blue, which says nothing about an object
that is already blue. Add a halo drawn just outside the selected objects
instead: a fixed colour against the background, so it reads whatever the
element is painted.

The mask pass reuses the main shader module and pipeline layout and shares
the main depth buffer read-only, so the halo follows the selection as
visible -- an occluded object contributes nothing. A separable dilation
widens the mask into inner and outer rings, composited after the edge pass
so the edge multiply does not darken it.

On by default; IfcViewer.setSelectionOutline(false) gets the tint alone.

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

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

@@ -0,0 +1,45 @@
+// This file was generated with the assistance of an AI coding tool.
+//
+// Selection silhouette outline. The renderer's blue selection tint says
+// nothing about an object that is already blue, so the halo is the cue that
+// has to survive that case: this paints the whole sample the exact palette
+// blue that collides, selects one element, and checks the canvas changes.
+import { test, expect } from '@playwright/test';
+
+const shot = (page) => page.locator('#viewer-canvas').screenshot();
+
+test('halo reads on an object painted the selection tint colour', async ({ page }) => {
+  const gpuErrors = [];
+  page.on('console', (msg) => {
+    if (/Uncaptured WebGPU error|is invalid|Not enough memory left/i.test(msg.text()))
+      gpuErrors.push(msg.text());
+  });
+  page.on('pageerror', (e) => gpuErrors.push('pageerror: ' + e.message));
+
+  await page.goto('/scripting.html');
+  await page.waitForFunction(() => !!(window.viewer && window.viewer.isLive()),
+                             null, { timeout: 30_000 });
+  await page.waitForTimeout(1500);
+
+  const n = await page.evaluate(async () => {
+    const v = window.viewer;
+    const objs = await v.getObjects();
+    v.setColor(objs, '#3987e5');
+    v.setSelection([objs[0].objectId]);
+    return objs.length;
+  });
+  expect(n).toBeGreaterThan(0);
+  await page.waitForTimeout(600);
+  const withOutline = await shot(page);
+
+  await page.evaluate(() => window.viewer.setSelectionOutline(false));
+  await page.waitForTimeout(600);
+  const withoutOutline = await shot(page);
+
+  expect(
+    Buffer.compare(withOutline, withoutOutline),
+    'the outline toggle changed nothing — halo never drew',
+  ).not.toBe(0);
+
+  expect(gpuErrors, gpuErrors.join('\n')).toEqual([]);
+});