feat(view): round bar bends at the centre-line radius; bend factors as inputs on ③

Bars drawn through several points are one tube each, with every bend rounded by R·tan(φ/2) at the centre-line radius
(inner factor × d + d/2). The inside-radius factors (D16 and below, D19–D25, D29–D35, D38 and above) are inputs in 공통
설정, with KDS-based defaults 2 / 3 / 4 / 5. The size-class limits sit in one place (SIZE_CLASS), which also fixes
D25 (25.4) being read as the D29–D35 class in hookLength. The helper bendDeduction gives the centre-line deduction for
the length option, which is not yet applied to the 철근표 lengths.

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

tests/rebar-grade.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest';
 import { buildPier } from '../src/core/geometry';
-import { DEFAULT_MATERIAL, fyOf, isAssumedMaterial, materialFrom } from '../src/core/rebar/common';
+import { DEFAULT_BEND, DEFAULT_MATERIAL, fyOf, isAssumedMaterial, materialFrom } from '../src/core/rebar/common';
 import { rebarSummary } from '../src/core/rebar/summary';
 import { loadP1 } from './helpers';
 
@@ -19,7 +19,7 @@ describe('③ 철근 강도: 규격 접두어별 입력 (승인 2026-10-23)', ()
   });
 
   it('an older file that stored fyH / fyS is read into the grade map', () => {
-    expect(materialFrom({ fck: 35, fyH: 400, fyS: 400 })).toEqual({ fck: 35, grade: { D: 300, H: 400, S: 400 } });
+    expect(materialFrom({ fck: 35, fyH: 400, fyS: 400 })).toEqual({ fck: 35, grade: { D: 300, H: 400, S: 400 }, bend: DEFAULT_BEND });
   });
 
   it('a value outside SD300 · SD400 · SD500 falls back to the default for its prefix', () => {
@@ -34,3 +34,18 @@ describe('③ 철근 강도: 규격 접두어별 입력 (승인 2026-10-23)', ()
     expect(same).toBe(base);
   });
 });
+
+import { bendDeduction, bendRadius } from '../src/core/rebar/common';
+
+describe('굽힘 반경 (승인 2026-10-23, 입력)', () => {
+  it('centre-line radius = inner factor × d + d/2, by size class', () => {
+    expect(bendRadius(25.4, DEFAULT_BEND)).toBeCloseTo(3 * 25.4 + 12.7, 6); // H25: main25
+    expect(bendRadius(28.6, DEFAULT_BEND)).toBeCloseTo(4 * 28.6 + 14.3, 6); // H29: main35
+    expect(bendRadius(15.9, DEFAULT_BEND)).toBeCloseTo(2 * 15.9 + 7.95, 6); // H16: small16
+  });
+
+  it('a 90° bend loses (2 − π/2)·R of centre line against the sharp corner', () => {
+    const R = 100;
+    expect(bendDeduction(Math.PI / 2, R)).toBeCloseTo((2 - Math.PI / 2) * R, 6);
+  });
+});