feat(core,view): footing rebar B1–B12 (plan C5) and the ③ rebar screen v1

- Footing (확대기초철근 (1).md): grid counts from W and L, layer heights,
  B1–B4 and B5–B8 counts and lengths (B2 13646, B6 11396), B10-1/B10-2,
  B11 (60) and B12 (47) with their lengths. B9 is not computed yet.
- rebarSummary: one 철근표 per pier from the model (bearing, coping, column,
  footing), with alerts: RB-F08 (B2 needs a splice, Q-B6), RB-F09 (B9 not
  computed).
- ③ screen v1: stage nav button, member tabs (받침블럭 · 코핑 · 기둥 · 확대기초),
  rebar table with 번호 · 규격 · 개수 · 길이 · 비고, alert list, and the open
  questions (Q-B5, Q-B6, Q-C9). Drawings and the 강조 연동 are not in v1.
- Tests: footing values from the document; P1 summary totals; warnings.
- Not verified in a browser.

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

tests/rebar-footing.test.ts

@@ -0,0 +1,70 @@
+import { describe, expect, it } from 'vitest';
+import { assemblyBars, bottomBars, crossBars, grid, layers, legB2, sideBars } from '../src/core/rebar/footing';
+
+// P1 footing: L 10400 × W 8150 × H 2000.
+const L = 10400;
+const W = 8150;
+const H = 2000;
+
+describe('확대기초 철근 (확대기초철근 (1).md, plan C5)', () => {
+  it('grid across W: n 61, G 7625, e 162.5, 62 positions (§3.3)', () => {
+    const g = grid(W);
+    expect(g.n).toBe(61);
+    expect(g.G).toBe(7625);
+    expect(g.e).toBeCloseTo(162.5, 6);
+    expect(g.positions).toBe(62);
+  });
+
+  it('배근도 재현 grid: W 7800 → n 58, e 175', () => {
+    const g = grid(7800);
+    expect(g.n).toBe(58);
+    expect(g.e).toBeCloseTo(175, 6);
+  });
+
+  it('layer heights: B5 1900, B1 1873, B2·B4 150, B6·B8 177, B3 250, B7 277', () => {
+    const z = layers(H);
+    expect(z.B5).toBe(1900);
+    expect(z.B1).toBeCloseTo(1873, 6);
+    expect(z.B24).toBe(150);
+    expect(z.B68).toBeCloseTo(177, 6);
+    expect(z.B3).toBe(250);
+    expect(z.B7).toBeCloseTo(277, 6);
+  });
+
+  it('B2 leg 1723, B2 length 13646 (over the 12000 bar, splice needed)', () => {
+    expect(legB2(H)).toBeCloseTo(1723, 6);
+    const b2 = bottomBars(L, W, H).find((b) => b.name === 'B2')!;
+    expect(b2.length).toBeCloseTo(13646, 0);
+    expect(b2.count).toBe(31);
+  });
+
+  it('B1–B4 counts 31 / 31 / 31 / 31 and B4 length 10200', () => {
+    const bars = bottomBars(L, W, H);
+    expect(bars.map((b) => b.count)).toEqual([31, 31, 31, 31]);
+    expect(bars.find((b) => b.name === 'B4')!.length).toBeCloseTo(10200, 6);
+  });
+
+  it('B5–B8 counts 40 each (grid along L: n 79), B8 length 7950, B6 length 11396', () => {
+    const bars = crossBars(L, W, H);
+    expect(bars.map((b) => b.count)).toEqual([40, 40, 40, 40]);
+    expect(bars.find((b) => b.name === 'B8')!.length).toBeCloseTo(7950, 6);
+    expect(bars.find((b) => b.name === 'B6')!.length).toBeCloseTo(11396, 0);
+  });
+
+  it('B10-1 18 bars ≈ 10748, B10-2 18 bars ≈ 8494', () => {
+    const [b101, b102] = sideBars(L, W);
+    expect(b101.count).toBe(18);
+    expect(b101.length).toBeCloseTo(10748, 0);
+    expect(b102.count).toBe(18);
+    expect(b102.length).toBeCloseTo(8494, 0);
+  });
+
+  it('B11 60 bars (20 / 20 / 20) ≈ 8359, B12 47 bars (16 / 15 / 16) ≈ 10613', () => {
+    const [b11, b12] = assemblyBars(L, W);
+    expect(b11.count).toBe(60);
+    // The document rounds the 90° hook to 246 mm; the formula gives 246.45, so B11 is 1.2 mm longer.
+    expect(Math.abs(b11.length - 8359)).toBeLessThan(1.5);
+    expect(b12.count).toBe(47);
+    expect(b12.length).toBeCloseTo(10613, 0);
+  });
+});