Fix GUI paste of CRLF/CR text: split all line endings

The ImGui Cmd+V/Ctrl+V paste handler split clipboard text only on '\n',
leaving '\r' inside each segment. Pasting text with CRLF (Windows) or bare
CR (classic-Mac / some macOS apps) line endings enqueued an InsertText arg
containing '\r', which the InsertText command rejects with "InsertText arg
must not contain newlines".

Extract the paste-to-commands logic into a pure, SDL-free helper
(PasteSplit.h) that treats '\n', '\r\n', and bare '\r' each as one line
break, and add unit tests covering all line-ending forms plus blank-line
and trailing-newline preservation.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -4,7 +4,7 @@ project(kte)
 include(GNUInstallDirs)
 
 set(CMAKE_CXX_STANDARD 20)
-set(KTE_VERSION "1.11.2")
+set(KTE_VERSION "1.11.3")
 
 # Default to terminal-only build to avoid SDL/OpenGL dependency by default.
 # Enable with -DBUILD_GUI=ON when SDL2/OpenGL/Freetype are available.
@@ -345,6 +345,7 @@ if (BUILD_TESTS)
             tests/test_smart_newline.cc
             tests/test_reflow_undo.cc
             tests/test_syntax_highlighting.cc
+            tests/test_paste_split.cc
 
             # minimal engine sources required by Buffer
             PieceTable.cc