mod: Phase 3 — Re-port unmerged upstream PRs

Re-applied upstream PRs not yet merged to upstream/master:

- #1055: Byte-level framebuffer writes (fillPhysicalHSpan*,
  optimized fillRect/drawLine/fillRectDither/fillPolygon)
- #1027: Word-width cache (FNV-1a, 128-entry) and hyphenation
  early exit in ParsedText for 7-9% layout speedup
- #1068: Already present in upstream — URL hyphenation fix
- #1019: Already present in upstream — file extensions in browser
- #1090/#1185/#1217: KOReader sync improvements — binary credential
  store, document hash caching, ChapterXPathIndexer integration
- #1209: OPDS multi-server — OpdsBookBrowserActivity accepts
  OpdsServer, directory picker for downloads, download-complete
  prompt with open/back options
- #857: Dictionary activities already ported in Phase 1/2
- #1003: Placeholder cover already integrated in Phase 2

Also fixed: STR_OFF i18n string, include paths, replaced
Epub::isValidThumbnailBmp with Storage.exists, replaced
StringUtils::checkFileExtension with FsHelpers equivalents.

Made-with: Cursor

lib/KOReaderSync/KOReaderDocumentId.h

@@ -42,4 +42,31 @@ class KOReaderDocumentId {
 
   // Calculate offset for index i: 1024 << (2*i)
   static size_t getOffset(int i);
+
+  // Hash cache helpers
+  // Returns the path to the per-book cache file that stores the precomputed hash.
+  // Uses the same directory convention as the Epub cache (/.crosspoint/epub_<hash>/).
+  static std::string getCacheFilePath(const std::string& filePath);
+
+  // Returns the cached hash if the file size and fingerprint match, or empty
+  // string on miss/invalidation.
+  //
+  // The fingerprint is derived from the file's modification timestamp.  We
+  // call `FsFile::getModifyDateTime` to retrieve two 16‑bit packed values
+  // supplied by the filesystem: one for the date and one for the time.  These
+  // are concatenated and represented as eight hexadecimal digits in the form
+  // <date><time> (high 16 bits = packed date, low 16 bits = packed time).
+  //
+  // The resulting string serves as a lightweight change signal; any modification
+  // to the file's mtime will alter the packed date/time combo and invalidate
+  // the cache entry.  Since the full document hash is expensive to compute,
+  // using the packed timestamp gives us a quick way to detect modifications
+  // without reading file contents.
+  static std::string loadCachedHash(const std::string& cacheFilePath, size_t fileSize,
+                                    const std::string& currentFingerprint);
+
+  // Persists the computed hash alongside the file size and fingerprint (the
+  // modification-timestamp token) used to generate it.
+  static void saveCachedHash(const std::string& cacheFilePath, size_t fileSize, const std::string& fingerprint,
+                             const std::string& hash);
 };