feat(rebar): P3 and P6 end at the coping middle level; e_top computed from it

The coping middle level is (Level underside + step top)/2. P3's top and P6's upper limit are this level, and e_top is the
step top minus it (P1: 1322, replacing the 1623 reverse-engineered value). On P1 P3 goes from 38 to 39 ties and P6 stays at
8 levels. With taper h2 = 3000 the middle level is 13275: P3 37 ties, P6 8 levels (32 bars), e_top 2072.

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

tests/column-model.test.ts

@@ -24,4 +24,15 @@ describe('기둥 철근 높이는 모델에서 (Hc · h_cop, 승인 2026-10-23)'
     // The P1 top is anchored to the step top (and C3), so it does not move with h2.
     expect(col.zTop1).toBeCloseTo(14862, 0);
   });
+
+  it('P3 and P6 end at the coping middle level: P1 e_top 1322; taper 3000 gives e_top 2072, P3 37 ties, P6 32 bars', () => {
+    const p1 = columnInputFor(buildPier(loadP1()));
+    expect(p1.eTop).toBeCloseTo(1322, 0);
+    const high = columnInputFor(buildPier({ ...loadP1(), copingH2Mm: 3000 }));
+    expect(high.eTop).toBeCloseTo(2072, 0);
+    const col = columnRebar(high);
+    expect(col.zTopT).toBeCloseTo(13275, 0);
+    expect(col.p3Count).toBe(37);
+    expect(col.p6Count).toBe(32);
+  });
 });