feat(core,view): per-row pile counts 열별 본수 (plan C4)

- A 열 is one row across the footing width; each has its own count along the
  length. Default floor((L − 1400) / 2250) + 1 per 열, as in spec §4.6.
- Counts are validated: one per 열, whole numbers 2–30. Spacing below the
  2250 mm minimum is shown as a warning, not refused (the plan's 6본 → 5@1800
  example).
- PierInput gains an optional pileRows; absent means the defaults, so existing
  files and the P1 results are unchanged.
- Change tracking compares arrays by value; 열별 본수 can be reverted and
  copied (copy item off by default).
- The footing card has −/+ per 열 with spacing, total, and the display string.
- Tests: defaults 4열 × 5본 = 20 and the 1단계 text; 6본 → 5@1800; limits;
  model-level 21 piles.
- Not verified in a browser.

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

index.html

@@ -296,6 +296,7 @@
               </li>
               <li><span class="cp-group">확대기초 · 말뚝</span>
                 <label class="chk"><input type="checkbox" data-item="pile-length" />말뚝 길이 <span class="muted small">지반마다 다름 · 기본 해제</span></label>
+                <label class="chk"><input type="checkbox" data-item="pile-rows" />열별 본수 <span class="muted small">기초 크기마다 다름 · 기본 해제</span></label>
                 <label class="chk disabled"><input type="checkbox" disabled />기초 형상 · 치수 · 버림 <span class="muted small">예정</span></label>
               </li>
               <li><span class="cp-group">기둥 · 받침 · 철근배근</span>