test(core): footing drawing-reproduction checks from the document's test table

- Leg offsets (B1 +25.4, B3 +22.25, B5 +28.6, B7 +23.85) and B11 / B12 heights
  (550 / 950 / 1350; 569.1 / 969.1 / 1369.1).
- Reproduction counts: W 7800 → B4 30 / B2 29; L 9350 → B8 36 / B6 35.
- Variables: W 8000 with s 150 → 50 steps, e 150; W 12000 → 92 steps.
- Splice cases: L 8700 → B2 11946 (no splice), L 12400 → B4 12200,
  W 8800 → B6 12046.
- Not covered: the 배근도 H 2200 case for B10 (the document's count is unclear).

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

src/core/rebar/footing.ts

@@ -37,7 +37,7 @@ export interface GridResult {
 }
 
 /** Grid along one side of the footing (§3.1). side = W for B1–B4, L for B5–B8. */
-export function grid(side: number, s = FOOTING_DEFAULTS.s, cSide = FOOTING_DEFAULTS.cSide, eMin = FOOTING_DEFAULTS.eMin): GridResult {
+export function grid(side: number, s: number = FOOTING_DEFAULTS.s, cSide: number = FOOTING_DEFAULTS.cSide, eMin: number = FOOTING_DEFAULTS.eMin): GridResult {
   const A = side - 2 * cSide;
   const n = Math.floor((A - 2 * eMin) / s);
   const G = n * s;
@@ -165,3 +165,30 @@ export function shearB9(L: number, W: number, H: number): { rows: number; perRow
   const lh = hookLength('stirrup135', d('H16'));
   return { rows: rowIndices.length, perRow, count, length: lv + 2 * lh };
 }
+
+/** Leg offsets from the grid line (§5.3, §6.3): the B1/B3 and B5/B7 legs sit beside the outer leg. */
+export function legOffsets(): { B1: number; B3: number; B5: number; B7: number } {
+  const dB2 = NOMINAL_DIAMETER.S25;
+  const dB1 = NOMINAL_DIAMETER.S25;
+  const dB3 = NOMINAL_DIAMETER.S19;
+  const dB6 = NOMINAL_DIAMETER.S29;
+  const dB5 = NOMINAL_DIAMETER.S29;
+  const dB7 = NOMINAL_DIAMETER.S19;
+  return {
+    B1: (dB2 + dB1) / 2,
+    B3: (dB2 + dB3) / 2,
+    B5: (dB6 + dB5) / 2,
+    B7: (dB6 + dB7) / 2,
+  };
+}
+
+/** B11/B12 heights (§8.1): B10 levels at 150 + k·s_v; the assembly levels are j = n, 2n, 3n (n = 2), and B12 sits d_B10 above B11. */
+export function assemblyHeights(): { B11: number[]; B12: number[] } {
+  const { cBottom } = FOOTING_DEFAULTS;
+  const sV = FOOTING_DEFAULTS.sV;
+  const n = FOOTING_DEFAULTS.n;
+  const js = [n, 2 * n, 3 * n];
+  const B11 = js.map((j) => cBottom + j * sV);
+  const B12 = B11.map((z) => z + NOMINAL_DIAMETER.S19);
+  return { B11, B12 };
+}

tests/rebar-footing.test.ts

@@ -76,3 +76,47 @@ describe('확대기초 철근 (확대기초철근 (1).md, plan C5)', () => {
     expect(b9.length).toBeCloseTo(2128, 0);
   });
 });
+
+import { assemblyHeights, legOffsets } from '../src/core/rebar/footing';
+
+describe('확대기초 배근도 재현 (확대기초철근 (1).md §13)', () => {
+  it('leg offsets: B1 +25.4, B3 +22.25, B5 +28.6, B7 +23.85', () => {
+    const o = legOffsets();
+    expect(o.B1).toBeCloseTo(25.4, 6);
+    expect(o.B3).toBeCloseTo(22.25, 6);
+    expect(o.B5).toBeCloseTo(28.6, 6);
+    expect(o.B7).toBeCloseTo(23.85, 6);
+  });
+
+  it('B11 at 550 / 950 / 1350 and B12 at 569.1 / 969.1 / 1369.1', () => {
+    const h = assemblyHeights();
+    expect(h.B11).toEqual([550, 950, 1350]);
+    expect(h.B12.map((z) => Math.round(z * 10) / 10)).toEqual([569.1, 969.1, 1369.1]);
+  });
+
+  it('배근도 재현 W 7800 → B4 30, B2 29 (odd count split)', () => {
+    const bars = bottomBars(10400, 7800, 2000);
+    expect(bars.find((b) => b.name === 'B4')!.count).toBe(30);
+    expect(bars.find((b) => b.name === 'B2')!.count).toBe(29);
+  });
+
+  it('배근도 재현 L 9350 → B8 36, B6 35', () => {
+    const bars = crossBars(9350, 8150, 2000);
+    expect(bars.find((b) => b.name === 'B8')!.count).toBe(36);
+    expect(bars.find((b) => b.name === 'B6')!.count).toBe(35);
+  });
+
+  it('variables: W 8000 with s 150 → 50 grid steps, e 150, B4 26 / B2 25; W 12000 → 92 steps, e 150', () => {
+    const g = grid(8000, 150);
+    expect(g.n).toBe(50);
+    expect(g.e).toBeCloseTo(150, 6);
+    expect(grid(12000).n).toBe(92);
+    expect(grid(12000).e).toBeCloseTo(150, 6);
+  });
+
+  it('splice cases: L 8700 → B2 11946 (no splice); L 12400 → B4 12200; W 8800 → B6 12046', () => {
+    expect(bottomBars(8700, 8150, 2000).find((b) => b.name === 'B2')!.length).toBeCloseTo(11946, 0);
+    expect(bottomBars(12400, 8150, 2000).find((b) => b.name === 'B4')!.length).toBeCloseTo(12200, 0);
+    expect(crossBars(10400, 8800, 2000).find((b) => b.name === 'B6')!.length).toBeCloseTo(12046, 0);
+  });
+});