feat(rebar): C12 in the taper band (h2) and C13 in the end band (h1), from the coping geometry

The split between C12 and C13 follows the end underside, not a fixed level: C12 rows lie between the Level underside and
the end underside (taper height h2), C13 rows between the end underside and the top (end height h1). The break is the end
underside + 127 mm, the taper line's start at the end face. P1 is unchanged (C12 41.288–42.288, C13 42.538–42.788). Tests
check the band rule for h1 and h2 changes.

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

src/view/rebar3d.ts

@@ -204,7 +204,7 @@ export function buildRebar3D(m: PierModel, common: Material = DEFAULT_MATERIAL,
     // and two ㄷ bars: a body across the width at x_w, 25.4 above the row, with legs 1525 inward along the faces.
     const sideFace = B / 2 - 0.127;
     const dSide = NOMINAL_DIAMETER.H25;
-    const levels = sideRowLevels(m.levelBottom);
+    const levels = sideRowLevels(m.levelBottom, m.endBottom);
     const side = copingSideBars(L * 1000, levels.zBottom, levels.zBreak, m.copingGeom);
     for (const [name, rows] of [['C12', side.c12], ['C13', side.c13]] as const) {
       for (const r of rows) {