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/view/ground-svg.ts

@@ -20,9 +20,15 @@ export function groundSectionSvg(input: PierInput, model: PierModel): string {
   const footBottom = Math.min(...model.footings.map((f) => f.bottom));
   const pileTips = model.piles.map((p) => p.bottom);
   const lowestRow = rows.length ? rows[rows.length - 1].bottomEl : ground - 1;
-  const water = input.groundWaterOn && input.groundWaterEl !== undefined && Number.isFinite(input.groundWaterEl) ? input.groundWaterEl : undefined;
+  // 하상: the 설계홍수위 (solid) and the 세굴선 replace the groundwater (dashed, 승인 2026-10-26).
+  const river = (input.groundPosition ?? '육상') === '하상';
+  const level = river
+    ? (Number.isFinite(input.floodLevelEl) ? input.floodLevelEl : undefined)
+    : (input.groundWaterOn && Number.isFinite(input.groundWaterEl) ? input.groundWaterEl : undefined);
+  const scour = river && Number.isFinite(input.scourDepthM) && (input.scourDepthM as number) >= 0 ? (input.scourDepthM as number) : undefined;
+  const scourEl = scour !== undefined ? ground - scour : undefined;
 
-  const yMin = Math.min(footBottom, lowestRow, ...pileTips, water ?? ground) - 1;
+  const yMin = Math.min(footBottom, lowestRow, ...pileTips, level ?? ground, scourEl ?? ground) - 1;
   const yMax = Math.max(ground, footTop, input.copingTopEl) + 0.5;
   const scale = PLOT_H / (yMax - yMin);
   const y = (el: number) => TOP_PAD + (yMax - el) * scale;
@@ -51,10 +57,17 @@ export function groundSectionSvg(input: PierInput, model: PierModel): string {
   parts.push(`<line x1="0" y1="${y(ground)}" x2="${WIDTH}" y2="${y(ground)}" stroke="#8A6D3B" stroke-width="1.5"></line>`);
   parts.push(`<text x="8" y="${y(ground) - 6}" font-size="12" fill="#18212B">지반고 ${ground.toFixed(3)}</text>`);
 
-  // Groundwater (only when the input is on and has a value).
-  if (water !== undefined) {
-    parts.push(`<line x1="0" y1="${y(water)}" x2="${WIDTH}" y2="${y(water)}" stroke="#2F6DB5" stroke-dasharray="6 4"></line>`);
-    parts.push(`<text x="8" y="${y(water) - 4}" font-size="12" fill="#123E70">지하수위 ${water.toFixed(3)}</text>`);
+  // Groundwater (육상, when on) as a dashed line; the 설계홍수위 (하상) as a solid line.
+  if (level !== undefined) {
+    const dash = river ? '' : ' stroke-dasharray="6 4"';
+    const label = river ? '설계홍수위 DHWL' : '지하수위';
+    parts.push(`<line x1="0" y1="${y(level)}" x2="${WIDTH}" y2="${y(level)}" stroke="#2F6DB5" stroke-width="${river ? 2 : 1}"${dash}></line>`);
+    parts.push(`<text x="8" y="${y(level) - 4}" font-size="12" fill="#123E70">${label} ${level.toFixed(3)}</text>`);
+  }
+  // 세굴선 (하상): the bed after scour.
+  if (scourEl !== undefined) {
+    parts.push(`<line x1="0" y1="${y(scourEl)}" x2="${WIDTH}" y2="${y(scourEl)}" stroke="#8A6D3B" stroke-dasharray="3 3"></line>`);
+    parts.push(`<text x="8" y="${y(scourEl) + 12}" font-size="12" fill="#5B4A2E">세굴선 ${scourEl.toFixed(3)}</text>`);
   }
 
   // Footing, column stub, and piles.