feat(core): lengths for coping, column and the B2 split (추천안)

- Coping: C9/C10 lower U = 1740 + 2h with h linear along the slope and 2300 on
  the Level (3522 / 6098 / 6249 / 6340 from the document); C12 (3717–10183),
  C13 11746, C14 3447.5, C15 3852, C16 4070.
- Column: P3 = C_t + 1.3·l_d(H19) lap (추천안 for Q-C6, lap 1146.6), P5 = C_t.
- Footing: B2 13646 is split into two pieces at a splice with 1.3·l_d(S25,
  SD500) (추천안 for Q-B6), piece ≈ 7776 each.
- Not done: C9/C11 cap (삿갓) lengths (Q-K5 open), P1/P2 per-line pieces.
- Tests against the document values; P1 summary updated.
- Not verified in a browser.

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

src/core/rebar/column.ts

@@ -1,4 +1,4 @@
-import { developmentLength, hookLength, NOMINAL_DIAMETER } from './common';
+import { developmentLength, hookLength, NOMINAL_DIAMETER, spliceLength } from './common';
 
 /**
  * 기둥 철근 C4 (plan C4). Source: RAG/기둥철근 (1).md, 2026-10-22 최종 정리. Type-1 원형 중실 기둥.
@@ -93,7 +93,15 @@ export function columnRebar(x: ColumnInput = COLUMN_P1) {
   const p6Hook = hookLength('tie90', dH16);
   const p6Length = p6Straight + 2 * p6Hook;
 
+  // P3 lap: Q-C6 is open; recommended = 1.3·l_d of H19 (fck 27, fy 400).
+  const lapP3 = spliceLength(dH19, 400, 27);
+  const p3Length = 2 * Math.PI * tieRadius + lapP3;
+  const p5Length = 2 * Math.PI * tieRadius;
+
   return {
+    p3Length: round(p3Length, 1),
+    p5Length: round(p5Length, 1),
+    lapP3: round(lapP3, 1),
     r1: round(r1),
     r2: round(r2),
     angleStepDeg: 360 / x.nDiv,