feat(core): bearing-block drawing check and coping rebar C3 (plan C2·C3)

- Drawing check (spec §6.3): an integrated 2500 × 1200 block gives 18@125 = 2250
  along (ends 50 + 75) and 7@125 = 875 across (ends 162.5).
- C3 coping rebar, from RAG/코핑철근.md (2026-10-22 final):
  - C10: 9 bars @300 at 4800–7200 (k_max 4, beyond the Level edge).
  - C9 / C11: 18 bars each @250 outward, ends at 300 and 11700 (end gap 300).
  - Refuses a coping too short for C9 (RB-K01).
  - Side bars C12 5 rows / C13 2 rows at the 42.538 break; C14 38 bars;
    C15 11 bars, C16 22 bars.
- spec §6.4 is older than 코핑철근.md (C10/C9/C11 layout, C12 row count, C14
  dimensions). This follows the final document; spec §6.4 needs an update.
- Not yet: C1–C8 main bars (shape and lengths), the C12/C13 bar lengths and
  shapes, the drawing check of the coping sheet, and the ③ screen.

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

src/core/rebar/bearing-block.ts

@@ -32,7 +32,7 @@ export interface BarRow {
 }
 
 /** Bars along one 1200 mm side: available = dim − 2·(cover + first gap); gaps = floor(available / spacing). */
-export function barsAcross(dim = BLOCK_REBAR.dim): { count: number; spacing: number; firstEnd: number } {
+export function barsAcross(dim: number = BLOCK_REBAR.dim): { count: number; spacing: number; firstEnd: number } {
   const available = dim - 2 * (BLOCK_REBAR.cover + BLOCK_REBAR.firstBarGap);
   const gaps = Math.floor(available / BLOCK_REBAR.spacing);
   const span = gaps * BLOCK_REBAR.spacing;
@@ -41,7 +41,7 @@ export function barsAcross(dim = BLOCK_REBAR.dim): { count: number; spacing: num
 }
 
 /** Rebar of one block; blockHeight is the bearing block height in mm (h1 front or h2 back, pad included). */
-export function blockBars(blockHeight: number, dim = BLOCK_REBAR.dim): BarRow[] {
+export function blockBars(blockHeight: number, dim: number = BLOCK_REBAR.dim): BarRow[] {
   const across = barsAcross(dim);
   const length = BLOCK_REBAR.straight + 2 * BLOCK_REBAR.legLength;
   return [