feat(cbz): pure chapter-window helpers (chapterIndexAt/chapterWin) with table tests

frontend/src/lib/virt.ts

@@ -76,3 +76,33 @@ export class PageHeights {
     return [a, b];
   }
 }
+
+/** 页号 → 所在章下标;无章(扁平包或单组)恒 0;首章 start 之前的散页归首章。 */
+export function chapterIndexAt(chs: { start: number }[] | null, page: number): number {
+  if (!chs || chs.length < 2) return 0;
+  return chs.reduce((acc, c, i) => (c.start <= page ? i : acc), 0);
+}
+
+export interface ChapterWindow {
+  start: number; // 窗口首页(含)——滚动/挂载下界
+  end: number; // 窗口末页(不含)——锁章时的滚动边界
+  mountEnd: number; // 挂载上界(含预读章),恒 ≤ count 且 ≥ end
+}
+
+/** 阅读窗口:连读或扁平包 = 全书;锁章 = 本章滚动、本章 + prefetch 章挂载。 */
+export function chapterWin(
+  chs: { start: number }[] | null,
+  count: number,
+  ci: number,
+  continuous: boolean,
+  prefetch: number,
+): ChapterWindow {
+  if (!chs || chs.length < 2 || continuous) return { start: 0, end: count, mountEnd: count };
+  const c = Math.max(0, Math.min(ci, chs.length - 1));
+  const end = chs[c + 1]?.start ?? count;
+  return {
+    start: c === 0 ? 0 : chs[c].start,
+    end,
+    mountEnd: chs[c + 1 + Math.max(0, prefetch)]?.start ?? count,
+  };
+}