feat(core): bearing-block drawing check and coping rebar C3 (plan C2·C3)

- Drawing check (spec §6.3): an integrated 2500 × 1200 block gives 18@125 = 2250
  along (ends 50 + 75) and 7@125 = 875 across (ends 162.5).
- C3 coping rebar, from RAG/코핑철근.md (2026-10-22 final):
  - C10: 9 bars @300 at 4800–7200 (k_max 4, beyond the Level edge).
  - C9 / C11: 18 bars each @250 outward, ends at 300 and 11700 (end gap 300).
  - Refuses a coping too short for C9 (RB-K01).
  - Side bars C12 5 rows / C13 2 rows at the 42.538 break; C14 38 bars;
    C15 11 bars, C16 22 bars.
- spec §6.4 is older than 코핑철근.md (C10/C9/C11 layout, C12 row count, C14
  dimensions). This follows the final document; spec §6.4 needs an update.
- Not yet: C1–C8 main bars (shape and lengths), the C12/C13 bar lengths and
  shapes, the drawing check of the coping sheet, and the ③ screen.

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

tests/rebar-bearing.test.ts

@@ -32,3 +32,18 @@ describe('받침블럭 철근 (spec §6.3, plan C2)', () => {
     expect(totals(rebar)).toEqual({ S1: 80, S2: 80 });
   });
 });
+
+describe('받침블럭 도면 재현 (spec §6.3)', () => {
+  it('integrated block 2500 × 1200: 18@125 = 2250 along (ends 50 + 75), 7@125 = 875 across (ends 50 + 113 ≈ 162.5)', () => {
+    const along = barsAcross(2500);
+    expect(along.spacing).toBe(125);
+    expect(along.count - 1).toBe(18);
+    expect((along.count - 1) * along.spacing).toBe(2250);
+    expect(along.firstEnd).toBeCloseTo(125, 6);
+
+    const across = barsAcross(1200);
+    expect(across.count - 1).toBe(7);
+    expect((across.count - 1) * across.spacing).toBe(875);
+    expect(across.firstEnd).toBeCloseTo(162.5, 6); // 50 + 75 + 37.5, printed as 50 + 113 in the drawing
+  });
+});