Add swap file journaling for crash recovery.

- Introduced `SwapManager` for buffering and writing incremental edits to sidecar `.kte.swp` files.
- Implemented basic operations: insertion, deletion, split, join, and checkpointing.
- Added recovery design doc (`docs/plans/swap-files.md`).
- Updated editor initialization to integrate `SwapManager` instance for crash recovery across buffers.

CMakeLists.txt

@@ -118,6 +118,7 @@ set(COMMON_SOURCES
         Command.cc
         HelpText.cc
         KKeymap.cc
+        Swap.cc
         TerminalInputHandler.cc
         TerminalRenderer.cc
         TerminalFrontend.cc
@@ -203,6 +204,7 @@ set(COMMON_HEADERS
         Command.h
         HelpText.h
         KKeymap.h
+        Swap.h
         InputHandler.h
         TerminalInputHandler.h
         Renderer.h