Journal insert_row as one record; keep the cursor in the buffer; fix ENABLE_ASAN

- Buffer::insert_row recorded its text and newline as two swap records
  after inserting both. A checkpoint triggered by the first (1 MiB of
  edits, or the checkpoint timer) snapshotted the newline already, and
  the second added it again on replay: recovery after undoing C-k C-k
  silently gained a blank line. It is now one record.
- Visual-line yank placed the cursor relative to its row assuming that
  row was inside the selection; after C-k a or Enter it was not, and the
  cursor ended past the end of a line or the buffer, where typed text was
  recorded for undo at a position it was not inserted at and could not
  be undone. The formula uses the nearest selected row, and run_handler
  now clamps the cursor to the buffer before and after every command.
- ENABLE_ASAN added -fsanitize=address to CMAKE_C_FLAGS only; the
  project is C++, so nothing was instrumented. Add it to
  CMAKE_CXX_FLAGS.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015rcVQiiWyTzoNrsvLSaAfB

CMakeLists.txt

@@ -22,6 +22,8 @@ option(ENABLE_ASAN "Enable AddressSanitizer for builds" OFF)
 if (ENABLE_ASAN)
     message(STATUS "ASan enabled")
     set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fsanitize=address -fno-omit-frame-pointer")
+    # The project is C++; without this only C sources were instrumented.
+    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=address -fno-omit-frame-pointer")
     # Ensure the sanitizer is linked too (especially important on some platforms)
     set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -fsanitize=address")
 endif ()