feat(import): input template download with three sheets (spec 2.3 step 4)

- buildTemplateWorkbook: 교각 · IP · 받침 sheets with the headers the parser
  reads; value cells left empty so no blank-name rows raise E05
- dialog button 입력 템플릿 다운로드 (.xlsx)
- tests: sheet order, parser recognition, write-and-read round trip

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

src/main.ts

@@ -5,6 +5,7 @@ import * as XLSX from 'xlsx';
 import { parseWorkbook, type SheetReport } from './core/parseWorkbook';
 import { buildPier, type PierModel } from './core/geometry';
 import { NOT_IMPLEMENTED_RULES, validateInput, validatePier } from './core/validate';
+import { buildTemplateWorkbook } from './core/template';
 import type { Issue, PierInput, PierType } from './core/types';
 import { buildPierScene, viewPosition, type PartName, type PierScene, type ViewKind } from './view/scene';
 
@@ -520,6 +521,9 @@ $<HTMLButtonElement>('regen').addEventListener('click', regenerateChecked);
 $<HTMLInputElement>('project-name').addEventListener('input', updateApply);
 
 $<HTMLButtonElement>('pick-file').addEventListener('click', () => $<HTMLInputElement>('file').click());
+$<HTMLButtonElement>('download-template').addEventListener('click', () => {
+  XLSX.writeFile(buildTemplateWorkbook(), '교각입력템플릿.xlsx');
+});
 $<HTMLInputElement>('file').addEventListener('change', async (ev) => {
   const file = (ev.target as HTMLInputElement).files?.[0];
   if (!file) return;