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/dms.test.ts

@@ -0,0 +1,13 @@
+import { describe, expect, it } from 'vitest';
+import { parseDms } from '../src/core/dms';
+
+describe('parseDms', () => {
+  it('converts the P1 sheet string to decimal degrees (spec §12: 75.9489°)', () => {
+    expect(parseDms(` 75°56'55.9"`)).toBeCloseTo(75.9489, 4);
+  });
+
+  it('returns null for malformed strings (E03)', () => {
+    expect(parseDms('75.9489')).toBeNull();
+    expect(parseDms(`75°61'00"`)).toBeNull();
+  });
+});