feat(rebar): P1 top = C3 − 100 with e_P1 computed; coupler rule keeps 2.5 m below the P1 top

e_P1 is no longer an input: P1 top = C3 (lowest coping top − 127 − 2·57.2 = 43.170) − 100 = 43.070, so e_P1 = step top −
43.070 = 485 mm. The P1 top moves from 15147 to 14862 mm; the top piece shortens by 285 mm. Couplers are placed at k·3000
with at least 2500 mm below the P1 top (a value to confirm). On P1 the coupler count stays at four (3, 6, 9, 12 m). P2 and
the tie top do not change.

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

tests/column-model.test.ts

@@ -10,7 +10,7 @@ describe('기둥 철근 높이는 모델에서 (Hc · h_cop, 승인 2026-10-23)'
     expect(input.hcop).toBeCloseTo(2644, 0);
     const col = columnRebar(input);
     expect(col.zTop2).toBeCloseTo(14539, 0);
-    expect(col.zTop1).toBeCloseTo(15147, 0);
+    expect(col.zTop1).toBeCloseTo(14862, 0);
   });
 
   it('taper h2 = 3000: Level underside 39.411, Hc 11203, P2 top 13039 (= Level underside + l_d)', () => {
@@ -21,7 +21,7 @@ describe('기둥 철근 높이는 모델에서 (Hc · h_cop, 승인 2026-10-23)'
     expect(input.hcop).toBeCloseTo(4144, 0);
     const col = columnRebar(input);
     expect(col.zTop2).toBeCloseTo(13039, 0);
-    // The P1 top is anchored to the step top, so it does not move with h2.
-    expect(col.zTop1).toBeCloseTo(15147, 0);
+    // The P1 top is anchored to the step top (and C3), so it does not move with h2.
+    expect(col.zTop1).toBeCloseTo(14862, 0);
   });
 });