feat(ground): load ground_model.json and apply the pier-centre ground EL

- ground-model.ts: read and check schema 1 of the Python ground model.
- ② 지반·지층 card: import the json, show the DTM value beside the excel
  value, and apply it only when the user presses the button (tracked as an
  override, revertible).
- groundEl becomes an editable, tracked input; the project file keeps the
  imported model and checks it on open.

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

src/core/changes.ts

@@ -19,6 +19,7 @@ export const EDITABLE_FIELDS = [
   { key: 'copingH2Mm', label: '테이퍼 높이 h2', unit: 'mm' },
   { key: 'levelWidthMm', label: 'Level 폭', unit: 'mm' },
   { key: 'pileLengthMm', label: '말뚝 길이', unit: 'mm' },
+  { key: 'groundEl', label: '지반고 EL', unit: 'm' },
 ] as const;
 
 /** Inputs tracked for overrides and pending changes: the numeric cards plus 열별 본수 (an array, compared by value). */
@@ -41,6 +42,7 @@ export const FIELD_LABELS: Record<EditableKey, string> = {
   copingH2Mm: '테이퍼 높이 h2',
   levelWidthMm: 'Level 폭',
   pileLengthMm: '말뚝 길이',
+  groundEl: '지반고 EL',
   pileRows: '열별 본수',
 };