fix(rebar): side rows on the 250 grid up to the 400 mm top margin, not a fixed count of seven

C12 and C13 come from one grid from the Level underside + 127 at 250 mm, up to top − 400 mm (상면 여유). The fixed count of
seven rows is kept only for callers that give no top. P1 is unchanged (C12 5, C13 2); a taper of 3000 mm now gives C12 11
and C13 2, and the C13 first row is always the last C12 row plus 250.

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

src/core/coping-geom.ts

@@ -24,6 +24,11 @@ export const SIDE_COVER_M = 0.127;
  *   C13 rows lie in the end band (end underside … top, height h1).
  * zBreak = end underside + 127 is where the taper line starts at the end face (P1: 42.538).
  */
-export function sideRowLevels(levelBottom: number, endBottom: number): { zBottom: number; zBreak: number } {
-  return { zBottom: levelBottom + SIDE_COVER_M + 0.25, zBreak: endBottom + SIDE_COVER_M };
+export function sideRowLevels(levelBottom: number, endBottom: number, top?: number): { zBottom: number; zBreak: number; zTop?: number } {
+  // Top margin (상면 여유) at least 400 mm (코핑철근.md §6.1): the highest side row lies at or below top − 0.4.
+  return { zBottom: levelBottom + SIDE_COVER_M + 0.25, zBreak: endBottom + SIDE_COVER_M, zTop: top === undefined ? undefined : top - SIDE_TOP_MARGIN_M };
 }
+
+/** Pitch of the side-row grid (m), and the top margin 400 mm (상면 여유). */
+export const SIDE_PITCH_M = 0.25;
+export const SIDE_TOP_MARGIN_M = 0.4;