feat(core): cap bars of C9–C11 and piece count (추천안 for Q-K5)

- Recommended l_cap = h − 100 (top cover), so l_cap stays below h (RB-K04).
  Cap length = 1740 + 2·l_cap. The shear rows show U and cap ranges.
- Piece count 6 per position (3 closed stirrups × U + cap): C9 + C10 + C11 gives
  270 pieces, as in the document.
- P1/P2 per-line pieces are shown from the document (기둥철근.md §3.2).
- Tests: cap length and the 270 check.
- Not verified in a browser.

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

src/core/rebar/coping.ts

@@ -95,3 +95,18 @@ export const SIDE_LENGTHS = { C12: [3717, 5333, 6950, 8567, 10183], C13: 11746 }
 
 /** C15 (거꾸로 된 U, 3852) and C16 (L형, 4070 / 4069) lengths, mm (코핑철근.md §7). */
 export const STEP_LENGTHS = { C15: 3852, C16: [4070, 4069] as const, C14: 3447.5 } as const;
+
+/**
+ * 삿갓 (upper cap) of C9–C11 (코핑철근.md §4.3). l_cap is open (Q-K5); recommended here: l_cap = h − c (top cover 100),
+ * so that l_cap stays below h (RB-K04). Cap length = 1740 + 2·l_cap.
+ */
+export const CAP = { flat: 1740, cover: 100 } as const;
+
+export function capLength(x: number, coping: number): number {
+  const h = shearUHeight(x, coping);
+  const lCap = h - CAP.cover;
+  return CAP.flat + 2 * lCap;
+}
+
+/** Pieces per position: 3 closed stirrups × (U + cap) = 6. Total 270 for C9 + C10 + C11 (document). */
+export const PIECES_PER_POSITION = 6;