feat(core): project file save/open with schema check (plan B3)

- serializeProject / parseProject for the .pzr.json format (spec §8,
  schemaVersion 1). Parsing refuses other versions and damaged piers, and
  names the first broken pier.
- The 3D model is not stored; it is regenerated on open.
- Tests: save → open equality without the source workbook; regenerated
  models equal for 20 piers; version and damage refusals.

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

tests/project.test.ts

@@ -0,0 +1,49 @@
+import { describe, expect, it } from 'vitest';
+import { buildPier } from '../src/core/geometry';
+import { parseProject, serializeProject, SCHEMA_VERSION, type ProjectFile } from '../src/core/project';
+import { loadP1 } from './helpers';
+
+function sampleProject(): ProjectFile {
+  const p1 = loadP1();
+  // 20 piers: P1 plus 19 variants with different names and coping widths, so the file has a realistic size.
+  const piers = Array.from({ length: 20 }, (_, i) => {
+    const input = i === 0 ? p1 : { ...p1, name: `P${i + 1}`, copingWidthMm: 3100 + i * 10 };
+    return { id: `pier-${i + 1}`, input };
+  });
+  return {
+    schemaVersion: SCHEMA_VERSION,
+    project: { name: '운곡교 교각', bridge: '운곡교', client: '', author: '', crs: 'KATEC', memo: '' },
+    source: { fileName: '운곡교P1입력.xlsx', sheets: ['교각', 'IP', '받침'] },
+    piers,
+    history: [{ version: '0.1.00', note: '최초 저장', at: '2026-10-22T00:00:00Z' }],
+  };
+}
+
+describe('project file round trip (spec §8, plan B3)', () => {
+  it('save → open gives the same inputs, without the source workbook', () => {
+    const original = sampleProject();
+    const text = serializeProject(original);
+    const reopened = parseProject(text);
+    expect(reopened).toEqual(original);
+  });
+
+  it('regenerated models match the originals after reopening (20 piers)', () => {
+    const original = sampleProject();
+    const reopened = parseProject(serializeProject(original));
+    original.piers.forEach((p, i) => {
+      expect(buildPier(reopened.piers[i].input)).toEqual(buildPier(p.input));
+    });
+  });
+
+  it('refuses another schema version instead of loading it', () => {
+    const text = serializeProject({ ...sampleProject(), schemaVersion: 99 });
+    expect(() => parseProject(text)).toThrow('지원하지 않는 파일 버전');
+  });
+
+  it('refuses a damaged file and names the broken pier', () => {
+    const project = sampleProject();
+    const broken = { ...project, piers: [project.piers[0], { id: 'pier-2', input: null }] };
+    expect(() => parseProject(JSON.stringify(broken))).toThrow('교각 2');
+    expect(() => parseProject('{ not json')).toThrow('JSON');
+  });
+});