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>

src/core/project.ts

@@ -0,0 +1,59 @@
+import type { PierInput } from './types';
+
+/** Project file schema (spec §8). Bump only with a migration. */
+export const SCHEMA_VERSION = 1;
+
+export interface ProjectInfo {
+  name: string;
+  bridge: string;
+  client: string;
+  author: string;
+  crs: string;
+  memo: string;
+}
+
+export interface ProjectPier {
+  id: string;
+  /** Input as read from the spreadsheet. The 3D model is not stored; it is regenerated on open (spec §8). */
+  input: PierInput;
+}
+
+export interface ProjectFile {
+  schemaVersion: number;
+  project: ProjectInfo;
+  source: { fileName: string; sheets: string[] };
+  piers: ProjectPier[];
+  history: { version: string; note: string; at: string }[];
+}
+
+/** Serialises a project to the pretty-printed JSON stored in <교량명>_교각설계.pzr.json. */
+export function serializeProject(p: ProjectFile): string {
+  return `${JSON.stringify(p, null, 2)}\n`;
+}
+
+/**
+ * Parses and checks a project file. Throws with a message that names the first problem, so a damaged file is
+ * refused instead of being half-loaded.
+ */
+export function parseProject(text: string): ProjectFile {
+  let raw: unknown;
+  try {
+    raw = JSON.parse(text);
+  } catch {
+    throw new Error('프로젝트 파일이 JSON으로 읽히지 않습니다.');
+  }
+  const obj = raw as Partial<ProjectFile>;
+  if (!obj || typeof obj !== 'object') throw new Error('프로젝트 파일 형식이 아닙니다.');
+  if (obj.schemaVersion !== SCHEMA_VERSION) {
+    throw new Error(`지원하지 않는 파일 버전입니다 (schemaVersion ${String(obj.schemaVersion)}, 지원: ${SCHEMA_VERSION}).`);
+  }
+  if (!Array.isArray(obj.piers)) throw new Error('piers 목록이 없습니다.');
+  obj.piers.forEach((p, i) => {
+    if (!p || typeof p.id !== 'string' || !p.input) throw new Error(`교각 ${i + 1}: id 또는 input이 없습니다.`);
+    const input = p.input;
+    if (typeof input.name !== 'string' || !Array.isArray(input.ips) || !Array.isArray(input.bearings)) {
+      throw new Error(`교각 ${i + 1}(${String(input.name)}): 입력 형식이 맞지 않습니다.`);
+    }
+  });
+  return obj as ProjectFile;
+}