feat(core): parse P1 workbook and build T-type pier shape

- parse Excel input by header strings (pier, IP, bearing tables)
- DMS azimuth parser (E03), input validation (E01, E02)
- pure geometry: axis, bearings, stepped coping, column, footing, piles, blinding
- warnings W01, W02, W05, W07, I01
- regression tests against spec.md section 12 (P1)

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

tests/helpers.ts

@@ -0,0 +1,19 @@
+import { readFileSync } from 'node:fs';
+import { fileURLToPath } from 'node:url';
+import * as XLSX from 'xlsx';
+import { parseWorkbook } from '../src/core/parseWorkbook';
+import type { PierInput } from '../src/core/types';
+
+const P1_PATH = fileURLToPath(new URL('../public/samples/P1.xlsx', import.meta.url));
+
+export function loadWorkbook(path: string = P1_PATH): XLSX.WorkBook {
+  return XLSX.read(readFileSync(path), { type: 'buffer' });
+}
+
+export function loadP1(): PierInput {
+  const { piers, issues } = parseWorkbook(loadWorkbook());
+  if (issues.length) throw new Error(`P1 parse issues: ${JSON.stringify(issues)}`);
+  const p1 = piers.find((p) => p.name === 'P1');
+  if (!p1) throw new Error('P1 not found');
+  return p1;
+}