feat(view): add 3D pier viewer and P1 sample page

- three.js scene in the pier frame (pivot at first column centre)
- coping as extruded elevation outline; columns, footings, piles, blinding, bearing blocks and pads
- four view presets, per-part visibility toggles
- workbook load (sample or file), result table, issue list and undetermined defaults
- frame orientation test and bounding-box test

Co-Authored-By: Claude Haiku 5.5 <noreply@anthropic.com>

tests/scene.test.ts

@@ -0,0 +1,51 @@
+import * as THREE from 'three';
+import { describe, expect, it } from 'vitest';
+import { buildPier } from '../src/core/geometry';
+import { buildPierScene, frameMatrix } from '../src/view/scene';
+import { loadP1 } from './helpers';
+
+const model = buildPier(loadP1());
+const M = frameMatrix(model);
+
+/** Scene coordinates of a national-grid point at EL 0 (x = E − pivot E, z = −(N − pivot N)). */
+function sceneOf(e: number, n: number, el: number): THREE.Vector3 {
+  return new THREE.Vector3(e - model.pivot.e, el, -(n - model.pivot.n));
+}
+
+describe('3D frame (view/scene.ts)', () => {
+  it('is a proper rotation (determinant +1) so faces keep their winding', () => {
+    expect(M.determinant()).toBeCloseTo(1, 9);
+  });
+
+  it('maps local (along-axis s, across n, EL) to the national-grid position', () => {
+    // Block 1 on the front row: its centre in grid coordinates must match the local-to-grid formula.
+    const b = model.blocks[0];
+    const grid = {
+      e: model.bp1.e + model.u.e * b.s + model.nrm.e * b.n,
+      n: model.bp1.n + model.u.n * b.s + model.nrm.n * b.n,
+    };
+    const mapped = new THREE.Vector3(b.s, b.bottom, b.n).applyMatrix4(M);
+    const expected = sceneOf(grid.e, grid.n, b.bottom);
+    expect(mapped.distanceTo(expected)).toBeLessThan(1e-6);
+  });
+
+  it('bounding box spans pile tip (2.208 m) to the highest pad top (back bearing 5, 43.898 m = sheet E41)', () => {
+    const scene = buildPierScene(model);
+    const box = new THREE.Box3().setFromObject(scene.root);
+    expect(box.min.y).toBeCloseTo(2.208, 2);
+    expect(box.max.y).toBeCloseTo(43.898, 2);
+  });
+
+  it('builds the expected part counts for P1', () => {
+    const scene = buildPierScene(model);
+    const count = (key: keyof typeof scene.parts) => scene.parts[key].children.length;
+    expect(count('coping')).toBe(1);
+    expect(count('columns')).toBe(1);
+    expect(count('footings')).toBe(1);
+    expect(count('piles')).toBe(20);
+    expect(count('blocks')).toBe(10);
+    expect(count('pads')).toBe(10);
+    expect(count('blinding')).toBe(1);
+    expect(count('ground')).toBe(1);
+  });
+});