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/rebar-column.test.ts

@@ -31,12 +31,12 @@ describe('기둥 철근 C4 (기둥철근.md, plan C4)', () => {
     expect(c.tieCircumference).toBeCloseTo(5657.7, 1);
   });
 
-  it('P3: l_e = 2340, 38 ties from 2.340 at @300 (top 13.440)', () => {
-    expect(c.zTopT).toBe(13724);
+  it('P3: l_e = 2340, 39 ties from 2.340 at @300, top at the coping middle level 14.025 (승인 2026-10-23)', () => {
+    expect(c.zTopT).toBe(14025);
     expect(c.lE).toBe(2340);
-    expect(c.p3Count).toBe(38);
+    expect(c.p3Count).toBe(39);
     expect(c.p3[0]).toBe(2340);
-    expect(c.p3[c.p3.length - 1]).toBe(13440);
+    expect(c.p3[c.p3.length - 1]).toBe(13740);
   });
 
   it('P5: 19 ties @120 from 2.220 down to 0.060', () => {