feat(ground): 하상 selectable; 설계홍수위 and 세굴 깊이 for 하상

- shape-options: a per-option "confirmed" flag; 하상 is confirmed, 해상 is not.
- The 위치 chips store the choice in the pier's draft; 선택 shows it.
- 하상: the groundwater inputs give way to 설계홍수위 DHWL (floodLevelEl) and
  세굴 깊이 (scourDepthM). The groundwater value is kept (남겨두기).
- Checks: G07 DHWL missing, G08 DHWL below the 지반고, G09 scour negative or
  the scour line below the footing bottom. The groundwater check is skipped for 하상.
- Section: the DHWL as a solid line and the 세굴선 dashed.

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

src/core/shape-options.ts

@@ -8,6 +8,8 @@ export interface ShapeOption {
   label: string;
   /** 굵은 글씨 in the spec: the default that the 1단계 model builds. */
   isDefault: boolean;
+  /** The rule for this option is confirmed (승인 2026-10-26: 하상), so it can be chosen even if it is not the default. */
+  confirmed?: boolean;
 }
 
 export interface ShapeGroup {
@@ -17,6 +19,7 @@ export interface ShapeGroup {
 
 const d = (label: string): ShapeOption => ({ label, isDefault: true });
 const x = (label: string): ShapeOption => ({ label, isDefault: false });
+const ok = (label: string): ShapeOption => ({ label, isDefault: false, confirmed: true });
 
 export const SHAPE_GROUPS: Record<MemberId, ShapeGroup[]> = {
   bearing: [{ title: '배치', options: [d('분리'), x('일체')] }],
@@ -34,14 +37,14 @@ export const SHAPE_GROUPS: Record<MemberId, ShapeGroup[]> = {
     { title: '말뚝 종류', options: [d('강관'), x('PHC'), x('복합')] },
     { title: '경사말뚝', options: [d('없음'), x('있음')] },
   ],
-  ground: [{ title: '위치', options: [d('육상'), x('하상'), x('해상')] }],
+  ground: [{ title: '위치', options: [d('육상'), ok('하상'), x('해상')] }],
 };
 
 export const UNDEFINED_REASON = '규칙 미정 (Q6 확정 전 선택 불가)';
 
-/** Options currently selectable. Defaults are the only choices today. */
+/** Options currently selectable: the defaults and the options whose rule is confirmed. */
 export function isSelectable(option: ShapeOption): boolean {
-  return option.isDefault;
+  return option.isDefault || option.confirmed === true;
 }
 
 /** The default summary of a member, e.g. "교좌면 직각 · 하면 직선". */