feat(ground): N value per layer row (text, blow count or 타수/관입량 mm)

- StrataRow.nValue: text. "23" is a blow count (0–100), "50/5" a refusal
  (50 blows for 5 mm, 1–300 mm). Empty means not known yet.
- G06 (warning): a malformed N value. Strict, because the values will feed the
  bearing-capacity and liquefaction checks.
- The layer table has an N값 column; the project file checks that the rows
  are well formed.

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

src/core/ground.ts

@@ -71,3 +71,44 @@ export function validatePileEmbedment(input: PierInput, model: PierModel, issues
     });
   }
 }
+
+/** N값 as read from the text: a blow count, or a refusal "타수/관입량" with the penetration in mm. */
+export type NValue = { kind: 'empty' } | { kind: 'n'; n: number } | { kind: 'refusal'; blows: number; mm: number } | { kind: 'invalid' };
+
+/** Largest blow count and penetration accepted (표준관입시험 의 일반 범위; 연타 한계 50 blows / 300 mm). */
+export const N_MAX = 100;
+export const PENETRATION_MAX_MM = 300;
+
+/** Reads an N value. "" is empty; "23" is 23 blows; "50/5" is 50 blows for 5 mm (관입량 mm). */
+export function parseNValue(text: string | undefined): NValue {
+  const v = (text ?? '').trim();
+  if (v === '') return { kind: 'empty' };
+  if (/^\d+$/.test(v)) {
+    const n = Number(v);
+    return n <= N_MAX ? { kind: 'n', n } : { kind: 'invalid' };
+  }
+  const m = /^(\d+)\/(\d+)$/.exec(v);
+  if (m) {
+    const blows = Number(m[1]);
+    const mm = Number(m[2]);
+    if (blows >= 1 && blows <= N_MAX && mm >= 1 && mm <= PENETRATION_MAX_MM) return { kind: 'refusal', blows, mm };
+  }
+  return { kind: 'invalid' };
+}
+
+/**
+ * G06 (경고): an N value that is not a blow count or a refusal in the accepted range. Empty is allowed.
+ * The values are not used in the calculation yet; they will feed the 지지력 and 액상화 checks, so the format is strict.
+ */
+export function validateStrataNValues(input: PierInput, issues: Issue[]): void {
+  (input.strataRows ?? []).forEach((row, i) => {
+    if (parseNValue(row.nValue).kind === 'invalid') {
+      issues.push({
+        code: 'G06',
+        level: 'W',
+        pier: input.name,
+        message: `지층 ${i + 1}행 (${row.name}) N값 "${row.nValue}"의 형식이 맞지 않습니다. 타수(0–${N_MAX}) 또는 "타수/관입량 mm"(예: 50/5)으로 적으세요.`,
+      });
+    }
+  });
+}