feat(rebar): 철근재료표 by 강종 → 직경 → 번호 with unit weights, 할증 and Excel export

The quantity table sits below the 철근표 for the whole pier. Lines group by grade and nominal diameter, with
subtotals, unit weight 0.00617·d² (kg/m), the total in kg and ton, and a 할증 total. 할증 is an input in 공통 설정,
default 0 as an assumption. Couplers are counted, not weighed. Bars with no length yet (C9, C10, C11, C12, P1, P2) are
listed as missing and kept out of the totals. The table exports to Excel with SheetJS.

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

tests/project.test.ts

@@ -68,7 +68,7 @@ describe('project file round trip (spec §8, plan B3)', () => {
 describe('③ 공통 설정 in the project file', () => {
   it('fck and fy are kept through save and open; older files get the defaults through the app', () => {
     const p = sampleProject();
-    const withCommon = { ...p, common: { fck: 35, grade: { D: 300, H: 400, S: 500 }, bend: { small16: 2, main25: 3, main35: 4, main51: 5 } } };
+    const withCommon = { ...p, common: { fck: 35, grade: { D: 300, H: 400, S: 500 }, bend: { small16: 2, main25: 3, main35: 4, main51: 5 }, waste: 0 } };
     expect(parseProject(serializeProject(withCommon)).common).toEqual(withCommon.common);
     expect(parseProject(serializeProject(p)).common).toBeUndefined();
   });