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>

tests/rebar-shear-loops.test.ts

@@ -0,0 +1,22 @@
+import { describe, expect, it } from 'vitest';
+import { buildPier } from '../src/core/geometry';
+import { NOMINAL_DIAMETER } from '../src/core/rebar/common';
+import { PIECES_PER_POSITION, SHEAR_LOOP_STEP } from '../src/core/rebar/coping';
+import { buildRebar3D } from '../src/view/rebar3d';
+import { loadP1 } from './helpers';
+
+const d = NOMINAL_DIAMETER.H25;
+
+describe('C9·C10·C11 폐합 (코핑철근.md §4 · 수정 2026-10-23)', () => {
+  it('the loop step leaves room for a 삿갓 (one bar beside its U) before the next loop\'s U', () => {
+    // Loop i: U at x_i, 삿갓 at x_i + d. Loop i+1: U at x_i + step. Clear if step − d ≥ d.
+    expect(SHEAR_LOOP_STEP - d).toBeGreaterThanOrEqual(d - 1e-9);
+  });
+
+  it('three 폐합 per position and 6 pieces each: the count and the 철근표 are unchanged', () => {
+    expect(PIECES_PER_POSITION).toBe(6);
+    const rb = buildRebar3D(buildPier(loadP1()));
+    // C10 has 9 positions, each with 3 loops of 6 bars (2 U legs, U base, 삿갓 top + 2 legs) = 6 bars per loop.
+    expect(rb.byName.get('C10')!.length).toBe(9 * 3 * 6);
+  });
+});