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.

HighlighterEngine.h

@@ -5,6 +5,10 @@
 #include <memory>
 #include <unordered_map>
 #include <vector>
+#include <mutex>
+#include <condition_variable>
+#include <atomic>
+#include <thread>
 
 #include "Highlight.h"
 #include "LanguageHighlighter.h"
@@ -29,6 +33,11 @@ public:
 
     bool HasHighlighter() const { return static_cast<bool>(hl_); }
 
+    // Phase 3: viewport-first prefetch and background warming
+    // Compute only the visible range now, and enqueue a background warm-around task.
+    // warm_margin: how many extra lines above/below to warm in the background.
+    void PrefetchViewport(const Buffer &buf, int first_row, int row_count, std::uint64_t buf_version, int warm_margin = 200) const;
+
 private:
     std::unique_ptr<LanguageHighlighter> hl_;
     // Simple cache by row index (mutable to allow caching in const GetLine)
@@ -40,6 +49,28 @@ private:
         StatefulHighlighter::LineState state;
     };
     mutable std::unordered_map<int, StateEntry> state_cache_;
+
+    // Track best known contiguous state row for a given version to avoid O(n) scans
+    mutable std::unordered_map<std::uint64_t, int> state_last_contig_;
+
+    // Thread-safety for caches and background worker state
+    mutable std::mutex mtx_;
+
+    // Background warmer
+    struct WarmRequest {
+        const Buffer *buf{nullptr};
+        std::uint64_t version{0};
+        int start_row{0};
+        int end_row{0}; // inclusive
+    };
+    mutable std::condition_variable cv_;
+    mutable std::thread worker_;
+    mutable std::atomic<bool> worker_running_{false};
+    mutable bool has_request_{false};
+    mutable WarmRequest pending_{};
+
+    void ensure_worker_started() const;
+    void worker_loop() const;
 };
 
 } // namespace kte