feat(rebar): add downward 90° hooks (135 mm) to P6 crossties in 3D rebar model

tests/rebar-column.test.ts

@@ -84,3 +84,19 @@ describe('RB-C07 P4 삭제 규칙 (기둥철근.md §6.2)', () => {
     expect(r.kept).toEqual([-60]);
   });
 });
+
+describe('기둥철근 P1·P2 마디와 바닥 (기둥철근.md §3.2)', () => {
+  it('bottom at −1697.5 (−H_f + z_B7 + (d_B7 + d_P1)/2), P1 pieces 4697.5 + 525 / 3000 ×3 / 3147', () => {
+    expect(c.zBot).toBeCloseTo(-1697.55, 1);
+    expect(c.p1Hook).toBe(525);
+    expect(c.p1Pieces.map((v) => Math.round(v))).toEqual([5223, 3000, 3000, 3000, 3147]);
+  });
+
+  it('P2 pieces 4847.5 / 3000 ×3 / 2389', () => {
+    expect(c.p2Pieces.map((v) => Math.round(v))).toEqual([4848, 3000, 3000, 3000, 2389]);
+  });
+
+  it('P6 hook 135 each, so the length is 2071 (1800.9 + 2 × 135)', () => {
+    expect(c.p6Length).toBeCloseTo(2070.9, 1);
+  });
+});