feat(core): B9 shear bars of the footing (plan C5)

- B9 (확대기초철근 (1).md §9): H16 with the 135° stirrup hook, on the odd grid
  (i_W = 1, 5, …), 16 rows, 20 per row (even rows start at i_L 1, odd at 3),
  320 bars, length l_v 1750 + 2 × 189 ≈ 2128.
- The rebar summary now lists B9; the alert about B9 is gone. Positions and
  hooks are the document's proposed rules (Q-B18, Q-B19), noted on the row.
- Tests: the document's B9 values; P1 summary has 320 and only the splice alert.
- Not verified in a browser.

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

src/core/rebar/footing.ts

@@ -145,3 +145,23 @@ export function assemblyBars(L: number, W: number): FootingBar[] {
     { name: 'B12', size: 'S16', count: b12, length: L - 2 * 139.75 + 2 * hook },
   ];
 }
+
+/**
+ * B9 전단철근 (확대기초철근 (1).md §9). Wraps B5 (top outer) and B2·B4 (bottom outer), H16 @500 staggered.
+ * Plane positions and hooks are proposed rules (Q-B18, Q-B19).
+ */
+export function shearB9(L: number, W: number, H: number): { rows: number; perRow: number[]; count: number; length: number } {
+  const nW = grid(W).n; // 61
+  const nL = grid(L).n; // 79
+  const rowIndices: number[] = [];
+  for (let i = 1; i <= nW; i += 4) rowIndices.push(i);
+  // Even-numbered rows start at i_L = 1, odd-numbered rows at i_L = 3 (2s stagger).
+  const perRow = rowIndices.map((_, r) => {
+    const start = r % 2 === 0 ? 1 : 3;
+    return Math.floor((nL - start) / 4) + 1;
+  });
+  const count = perRow.reduce((a, b) => a + b, 0);
+  const lv = layers(H).B5 - layers(H).B24; // 1750
+  const lh = hookLength('stirrup135', d('H16'));
+  return { rows: rowIndices.length, perRow, count, length: lv + 2 * lh };
+}