Add viewport-aware syntax prefetching and background warming.

- Added prefetching in both terminal and GUI renderers to optimize visible row highlights.
- Introduced background worker for offscreen highlight warming to improve scrolling performance.
- Refactored `HighlighterEngine` to manage thread-safety, caching, and stateful re-computation.
- Integrated changes into `HighlighterEngine`, `TerminalRenderer`, and `GUIRenderer`.
- Bumped version to 1.2.0 in preparation for the release.

CMakeLists.txt

@@ -4,7 +4,7 @@ project(kte)
 include(GNUInstallDirs)
 
 set(CMAKE_CXX_STANDARD 17)
-set(KTE_VERSION "1.1.2")
+set(KTE_VERSION "1.2.0")
 
 # Default to terminal-only build to avoid SDL/OpenGL dependency by default.
 # Enable with -DBUILD_GUI=ON when SDL2/OpenGL/Freetype are available.