fix(D03): 못 읽은 채 떠남 = 고침 없음 — 처음 깨끗한 값 = 빈 선 (PLAN 572-3)

- 프로젝트 없음 · 불러오기 실패 때 savedJson "" 라 dirty 참 → 떠남 셋 물음이 잘못 뜸(ORCA 실측)
- test_560_4 한 줄 덧

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

D03_PlanAnalysis/D03_PlanAnalysis_UI_Page.ts

@@ -87,7 +87,7 @@ export async function renderD03PlanAnalysis(root: HTMLElement): Promise<void> {
   let epsg = 0;
   let yearStart = new Date().getFullYear();
   let network: PlanNetwork = emptyNetwork(start);
-  let savedJson = "";
+  let savedJson = JSON.stringify(network); // 못 읽은 채 떠남 = 고침 없음
   let undoStack: PlanNetwork[] = [];
   let redoStack: PlanNetwork[] = [];
   let surface: PlanSurface | null = null;