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/copy.ts

@@ -21,7 +21,7 @@ export const COPY_ITEMS: Record<CopyItem, ItemRule> = {
   'coping-length': { label: '코핑 길이 L', keys: ['copingLengthMm'], defaultOn: false, pileItem: false },
   'pile-length': { label: '말뚝 길이', keys: ['pileLengthMm'], defaultOn: false, pileItem: true },
   'pile-rows': { label: '열별 본수', keys: ['pileRows'], defaultOn: false, pileItem: true },
-  'ground-position': { label: '지반 위치·지하수위', keys: ['groundPosition', 'groundWaterOn', 'groundWaterEl'], defaultOn: false, pileItem: false },
+  'ground-position': { label: '지반 위치·지하수위', keys: ['groundPosition', 'groundWaterOn', 'groundWaterEl', 'floodLevelEl'], defaultOn: false, pileItem: false },
 };
 
 export const DEFAULT_COPY_ITEMS: CopyItem[] = (Object.keys(COPY_ITEMS) as CopyItem[]).filter((k) => COPY_ITEMS[k].defaultOn);