feat(ground): 해상 selectable with MSL levels, 고조위·저조위, and checks

- Rules confirmed (승인 2026-10-26): datum MSL; 고조위 HWL and 저조위 LWL as
  two levels; 세굴 깊이 as for 하상; 파랑과 선박 하중 excluded from this scope.
- Checks: G13 levels missing, G10 해저면 not below 고조위, G11 저조위 not
  below 고조위, G14 pile wall at or below the corrosion allowance reference.
- CORROSION_ALLOWANCE_MM = 1 is a PLACEHOLDER reference (not in the RAG
  documents); the card and G14 mark it as to be confirmed.
- Card: the 해상 inputs; the section draws 고조위 (solid), 저조위 (dashed),
  and labels the ground line 해저면 EL.

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

tests/shape-options.test.ts

@@ -36,8 +36,9 @@ describe('형상 선택지 (spec §5.2, plan C5)', () => {
   });
 });
 
-it('하상 is the only confirmed ground option; 해상 stays refused until its rule is confirmed', () => {
+it('하상 and 해상 are confirmed ground options (승인 2026-10-26); 육상 is the default', () => {
   const ground = SHAPE_GROUPS.ground[0].options;
   expect(ground.find((o) => o.label === '하상')!.confirmed).toBe(true);
-  expect(() => choose('ground', 0, '해상')).toThrow('규칙 미정');
+  expect(ground.find((o) => o.label === '해상')!.confirmed).toBe(true);
+  expect(choose('ground', 0, '해상')).toBe('해상');
 });