fix(bearing): show only the outer block margins as C4 and C5

- A 일체 pair's inner sides face the other bearing, so they are not C4 or C5:
  they are null and shown as "—" in the 배치 규칙 table.
- The table headings now describe the 교축 margins (bearing to the outer block edge).

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

tests/bearing-layout.test.ts

@@ -28,6 +28,9 @@ describe('받침 배치 규칙 (승인 2026-10-26)', () => {
     // The 2500 block is centred on the pair midpoint, so the outer margins are 601 (within the 5 mm drawing tolerance of 600).
     expect(d.find((x) => x.no === 1 && x.row === '전열')!.leftMm).toBeCloseTo(600, -1);
     expect(d.find((x) => x.no === 6 && x.row === '후열')!.rightMm).toBeCloseTo(600, -1);
+    // the inner sides face the other bearing of the pair: no C4 or C5 there
+    expect(d.find((x) => x.no === 1 && x.row === '전열')!.rightMm).toBeNull();
+    expect(d.find((x) => x.no === 6 && x.row === '후열')!.leftMm).toBeNull();
   });
 
   it('the front and back bearings of one number are about 85 mm apart on P1, so 일체 is allowed', () => {