Remove GapBuffer and associated legacy implementation.

- Deleted `GapBuffer` class and its API implementations.
- Removed `AppendBuffer` selector and conditional `KTE_USE_PIECE_TABLE` macros.
- Eliminated legacy support in buffer APIs, file I/O, benchmarks, and correctness tests.
- Updated guidelines and comments to reflect PieceTable as the default and only buffer backend.

Buffer.h

@@ -9,10 +9,7 @@
 #include <vector>
 #include <string_view>
 
-#include "AppendBuffer.h"
-#ifdef KTE_USE_BUFFER_PIECE_TABLE
 #include "PieceTable.h"
-#endif
 #include "UndoSystem.h"
 #include <cstdint>
 #include <memory>
@@ -66,11 +63,7 @@ public:
 
 	[[nodiscard]] std::size_t Nrows() const
 	{
-#ifdef KTE_USE_BUFFER_PIECE_TABLE
 		return content_LineCount_();
-#else
-		return nrows_;
-#endif
 	}
 
 
@@ -86,7 +79,7 @@ public:
 	}
 
 
-	// Line wrapper backed by AppendBuffer (GapBuffer/PieceTable)
+	// Line wrapper backed by PieceTable
 	class Line {
 	public:
 		Line() = default;
@@ -183,7 +176,7 @@ public:
 		// minimal find() to support search within a line
 		[[nodiscard]] std::size_t find(const std::string &needle, const std::size_t pos = 0) const
 		{
-			// Materialize to std::string for now; Line is backed by AppendBuffer
+			// Materialize to std::string for now; Line is backed by PieceTable
 			const auto s = static_cast<std::string>(*this);
 			return s.find(needle, pos);
 		}
@@ -256,29 +249,21 @@ public:
 		}
 
 
-		AppendBuffer buf_;
+		PieceTable buf_;
 	};
 
 
 	[[nodiscard]] const std::vector<Line> &Rows() const
 	{
-#ifdef KTE_USE_BUFFER_PIECE_TABLE
 		ensure_rows_cache();
 		return rows_;
-#else
-		return rows_;
-#endif
 	}
 
 
 	[[nodiscard]] std::vector<Line> &Rows()
 	{
-#ifdef KTE_USE_BUFFER_PIECE_TABLE
 		ensure_rows_cache();
 		return rows_;
-#else
-		return rows_;
-#endif
 	}
 
 
@@ -477,13 +462,8 @@ private:
 	std::size_t rx_      = 0; // render x (tabs expanded)
 	std::size_t nrows_   = 0; // number of rows
 	std::size_t rowoffs_ = 0, coloffs_ = 0; // viewport offsets
-#ifdef KTE_USE_BUFFER_PIECE_TABLE
 	mutable std::vector<Line> rows_; // materialized cache of rows (without trailing newlines)
-#else
-	std::vector<Line> rows_; // buffer rows (without trailing newlines)
-#endif
-#ifdef KTE_USE_BUFFER_PIECE_TABLE
-	// When using the adapter, PieceTable is the source of truth.
+	// PieceTable is the source of truth.
 	PieceTable content_{};
 	mutable bool rows_cache_dirty_ = true; // invalidate on edits / I/O
 
@@ -492,7 +472,7 @@ private:
 
 	// Helper to query content_.LineCount() while keeping header minimal
 	std::size_t content_LineCount_() const;
-#endif
+
 	std::string filename_;
 	bool is_file_backed_   = false;
 	bool dirty_            = false;