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>

src/style.css

@@ -671,4 +671,10 @@ body { background: #eef1f4; font-family: 'Noto Sans KR', system-ui, sans-serif;
 .chip-opt[aria-pressed='true'] { background: #1f5fa8; border-color: #1f5fa8; color: #ffffff; font-weight: 700; }
 .chip-opt:disabled { color: #8a95a3; background: #f2f4f7; border-style: dashed; cursor: default; }
 .pier-items { max-height: 300px; overflow-y: auto; }
+/* 열별 본수 (C4) */
+.pile-rows { margin-top: 10px; display: grid; gap: 6px; }
+.pile-row { display: flex; flex-wrap: wrap; align-items: center; gap: 8px; }
+.pile-row .btn.small { min-width: 40px; }
+.pile-row.warn { color: #8a4b00; }
+.warn-text { color: #8a4b00; font-size: 12px; }