fix(core): address white-box review feedback

- Fix unicode surrogate pairs and emoji slicing via Array.from in ORP calculator
- Add slot smoothing constraint to pivot element to prevent horizontal jitter
- Implement adaptive timestamp compensation and visibilitychange auto-pause
- Slim storage layer by updating only STORE_META on progress changes with memory fallback
- Align EPUB chapter indices to token array indices instead of raw character offsets
- Support standalone emoji tokens via Extended_Pictographic regex
- Code-split JSZip via dynamic import to optimize initial bundle size

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

src/core/epub.ts

@@ -1,14 +1,23 @@
-import JSZip from 'jszip';
-import type { Chapter } from '../types';
+export interface EpubChapterItem {
+  id: string;
+  title: string;
+  charOffset: number;
+}
 
 export interface ParsedEpub {
   title: string;
   author: string;
   fullText: string;
-  chapters: Chapter[];
+  rawChapters: EpubChapterItem[];
 }
 
+/**
+ * Client-side EPUB parser with dynamic JSZip import to reduce initial bundle size.
+ */
 export async function parseEpubFile(file: File | Blob): Promise<ParsedEpub> {
+  // Dynamic import JSZip to split ~100KB off the initial load bundle
+  const { default: JSZip } = await import('jszip');
+
   const arrayBuffer = await file.arrayBuffer();
   const zip = await JSZip.loadAsync(arrayBuffer);
 
@@ -39,8 +48,11 @@ export async function parseEpubFile(file: File | Blob): Promise<ParsedEpub> {
   const opfDoc = domParser.parseFromString(opfXml, 'application/xml');
 
   // Metadata
-  const title = opfDoc.querySelector('title, dc\\:title')?.textContent?.trim() || (file instanceof File ? file.name.replace(/\.epub$/i, '') : '未命名书籍');
-  const author = opfDoc.querySelector('creator, dc\\:creator')?.textContent?.trim() || '未知作者';
+  const title =
+    opfDoc.querySelector('title, dc\\:title')?.textContent?.trim() ||
+    (file instanceof File ? file.name.replace(/\.epub$/i, '') : '未命名书籍');
+  const author =
+    opfDoc.querySelector('creator, dc\\:creator')?.textContent?.trim() || '未知作者';
 
   // Manifest: id -> href
   const manifestMap = new Map<string, string>();
@@ -48,7 +60,6 @@ export async function parseEpubFile(file: File | Blob): Promise<ParsedEpub> {
     const id = item.getAttribute('id');
     const href = item.getAttribute('href');
     if (id && href) {
-      // Decode URL encoding
       manifestMap.set(id, decodeURIComponent(href));
     }
   });
@@ -62,7 +73,7 @@ export async function parseEpubFile(file: File | Blob): Promise<ParsedEpub> {
     }
   });
 
-  const chapters: Chapter[] = [];
+  const rawChapters: EpubChapterItem[] = [];
   let fullText = '';
   let chapterIndex = 1;
 
@@ -70,7 +81,6 @@ export async function parseEpubFile(file: File | Blob): Promise<ParsedEpub> {
     const relativeHref = manifestMap.get(idref);
     if (!relativeHref) continue;
 
-    // Resolve full path inside zip
     const filePath = opfDir ? resolvePath(opfDir, relativeHref) : relativeHref;
     const chapterFile = zip.file(filePath);
     if (!chapterFile) continue;
@@ -87,7 +97,6 @@ export async function parseEpubFile(file: File | Blob): Promise<ParsedEpub> {
 
     // Extract text content cleanly
     const body = chapterDoc.body || chapterDoc.documentElement;
-    // Remove scripts and styles
     body.querySelectorAll('script, style, svg').forEach((el) => el.remove());
 
     const paragraphs: string[] = [];
@@ -105,10 +114,11 @@ export async function parseEpubFile(file: File | Blob): Promise<ParsedEpub> {
 
     const chapterText = paragraphs.join('\n\n');
     if (chapterText.trim().length > 0) {
-      chapters.push({
+      const startCharOffset = fullText.length;
+      rawChapters.push({
         id: `chap-${chapterIndex}`,
         title: chapterTitle,
-        startIndex: fullText.length,
+        charOffset: startCharOffset,
       });
 
       fullText += (fullText ? '\n\n' : '') + chapterText;
@@ -124,7 +134,7 @@ export async function parseEpubFile(file: File | Blob): Promise<ParsedEpub> {
     title,
     author,
     fullText,
-    chapters,
+    rawChapters,
   };
 }