fix(B06): 세월교 · 박스 유입 수평공간 시작 = 본체 바닥 높이 흙막이 전면(89-3)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EYJJRCfHfdCGNhmXTjQKV7

B06_Section/B06_Section_UI_Cross_Body_Guard.ts

@@ -271,3 +271,20 @@ export function withoutGuardedFill<T extends BoxLayout | FordLayout>(
     label: { ...ford.label, text: `${ford.label.text} L${Math.ceil(lengthM - 1e-6)}m` },
   } as T;
 }
+
+/** 세월교 · 박스 유입 흙막이 수평공간 시작 = 본체 바닥(세월교 바닥판 · 박스 저판 상면) 높이의
+ *  흙막이 전면 자리(89-3). 본체가 이 측점에 없으면 null(배관 규칙). */
+export function guardFloorStart(
+  section: CrossSection,
+  base: BodyGuardBase | null,
+  wall: WallLayout,
+): OffsetPoint | null {
+  const end = base?.ends.inlet;
+  if (!end) return null;
+  const slab = section.culvert?.body === "box" ? (section.box?.slab_thickness_m ?? 0) : 0;
+  const floor = end.invert + slab;
+  return {
+    offset: wall.points[2].offset + wall.outward * wall.lean * (wall.topJoint.elevation - floor),
+    elevation: floor,
+  };
+}

B06_Section/B06_Section_UI_Cross_Culvert_Geom.ts

@@ -25,6 +25,7 @@ import { buildRevetWallGeometry } from "./B06_Section_UI_Cross_Wall";
 import {
   bodyGuardAxes,
   bodyGuardBase,
+  guardFloorStart,
   bodyGuardHeight,
   fordPipeFit,
   mergeGuardTrim,
@@ -596,7 +597,7 @@ export function computeCulvertLayout(
     outletBottom: outletWall?.bottomBack.elevation ?? 0,
     basinStart: culvert.hidden_pipe
       ? inletWall
-        ? pipeCorners.inlet.bottom
+        ? (guardFloorStart(section, body, inletWall) ?? pipeCorners.inlet.bottom)
         : null
       : basinFillStart,
     basinBottom: culvert.hidden_pipe ? (inletWall?.bottomBack.elevation ?? 0) : basinFillBottom,