fix(rebar): stack the three C9·C10·C11 폐합 to the right so they do not overlap

The three 폐합 of one position were drawn at the same x, so their U bars overlapped. Loop i now sits at x + i·2d and its
삿갓 one bar (d) beside its U, so no U meets another loop's U or 삿갓. The 철근표 counts and lengths are unchanged.
The rule is geometry only; the spec (§4.3) needs the stacking note.

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

src/core/rebar/coping.ts

@@ -252,6 +252,12 @@ export function capLength(x: number, coping: number): number {
   return CAP.flat + 2 * lCap;
 }
 
+/**
+ * Step (mm) between the three 폐합 at one position, to the right (수정 2026-10-23): a 삿갓 sits one bar (d) beside its U,
+ * so the next loop's U starts two bars on. Loop i is then at x + i·step and its 삿갓 at x + i·step + d, with no overlap.
+ */
+export const SHEAR_LOOP_STEP = 2 * NOMINAL_DIAMETER.H25;
+
 /** Pieces per position: 3 closed stirrups × (U + cap) = 6. Total 270 for C9 + C10 + C11 (document). */
 export const PIECES_PER_POSITION = 6;