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-summary.test.ts

@@ -24,12 +24,12 @@ describe('③ 철근표 요약 (P1)', () => {
     expect(count('C16')).toBe(22);
   });
 
-  it('column: P1 230, P2 230, couplers 368, P3 38, P5 19, P4 8, P6 32', () => {
+  it('column: P1 230, P2 230, couplers 368, P3 39, P5 19, P4 8, P6 32', () => {
     expect(count('P1')).toBe(230);
     expect(count('P2')).toBe(230);
     expect(count('주철근 커플러')).toBe(368);
     expect(count('띠철근 커플러')).toBe(35);
-    expect(count('P3')).toBe(38);
+    expect(count('P3')).toBe(39);
     expect(count('P5')).toBe(19);
     expect(count('P4')).toBe(8);
     expect(count('P6')).toBe(32);