docs(rebar): note on the C12 row that the drawing's bottom 6th row is C7·C8 (no added bars)

The 250 grid runs from the Level underside + 127 for C12 and C13 together. The drawing's sixth C12 row at the bottom is the
bottom bar C7·C8 (코핑철근.md §6.1), so no bars are added and the counts stay the same.

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

src/core/rebar/summary.ts

@@ -196,7 +196,7 @@ export function rebarSummary(
   const c12Range = side.c12.length ? `${Math.round(side.c12[0].straight)}–${Math.round(side.c12[side.c12.length - 1].straight)}` : '—';
   // Per bar: 2 일자 (straight) and 2 ㄷ (hook body + legs), so 4 bars per row position.
   const sidePieces = (bars: { straight: number }[]) => bars.flatMap((b) => [{ length: b.straight, count: 2 }, { length: hook, count: 2 }]);
-  rows.push({ member: 'coping', name: 'C12', size: 'H25', count: 4 * side.c12.length, length: null, pieces: sidePieces(side.c12), note: `측면, 경사 하면에서 끊김, 일자 ${c12Range} (${2 * side.c12.length}) · ㄷ ${Math.round(hook)} (${2 * side.c12.length})` });
+  rows.push({ member: 'coping', name: 'C12', size: 'H25', count: 4 * side.c12.length, length: null, pieces: sidePieces(side.c12), note: `측면, 경사 하면에서 끊김, 일자 ${c12Range} (${2 * side.c12.length}) · ㄷ ${Math.round(hook)} (${2 * side.c12.length}) · 도면 6단 중 하면 단(EL ${(model.levelBottom + 0.127).toFixed(3)})은 하면 주철근 C7·C8로 계산 (문서 §6.1)` });
   rows.push({ member: 'coping', name: 'C13', size: 'H25', count: 4 * side.c13.length, length: null, pieces: sidePieces(side.c13), note: `측면, 끝면까지, 일자 (${2 * side.c13.length}) · ㄷ ${Math.round(hook)} (${2 * side.c13.length})` });
   // C14 / C15 / C16 are derived from the model (코핑철근.md §7–8); the document's 38 / 11 / 22 and 3852 / 4070 / 4069 are checked in the tests.
   const L14 = model.copingL * 1000;