Fix undo correctness, crash-recovery, and syntax highlighting bugs from full codebase review

Undo/redo:
- cmd_newline recorded the undo node's position after the cursor moved past
  the split point, so undoing Enter joined the wrong pair of lines.
- Backspace/Delete line-joins recorded UndoType::Newline, whose apply()
  semantics are inverted for a join (forward=split); add UndoType::JoinLines
  with correct forward/backward behavior.
- Regex replace-all, indent/unindent region, kill-to-eol, kill-line,
  kill-region, and delete-word-prev/next mutated the buffer via the
  no-undo-recording raw APIs; they now record undo, grouped atomically via a
  shared UndoGroupGuard.
- Plain-text replace-all with an empty replacement advanced the scan
  position one character too far, skipping adjacent/overlapping matches.

Crash recovery / memory safety:
- Buffer's move ctor/assignment never carried over swap_rec_,
  on_disk_identity_, or the visual-line-mode fields, so the crash-recovery
  journal silently stopped tracking a buffer whenever Editor's
  std::vector<Buffer> reallocated or shifted (e.g. opening/closing files).
  Added SwapManager::Rehome() plus Flush()-before-mutate in
  Editor::AddBuffer/CloseBuffer so the journal's Buffer* key and the
  background writer thread never reference a stale address.
- UndoTree leaked its entire node graph (including all edit text) on every
  buffer close/reload; added ~UndoTree() to free it.
- main.cc didn't restore the terminal if an exception escaped the run loop;
  added an RAII guard around Frontend::Shutdown().

Input handling:
- Terminal frontend used getch() instead of get_wch(), silently dropping
  non-ASCII keyboard input despite linking wide-char ncurses.
- KKeymap's C-k lookup switches on an already-lowercased key, making
  `case 'E'` unreachable dead code; C-k Shift-E silently aliased to C-k e.
- ImGui file picker wasn't modal: keystrokes typed while it was open still
  reached the buffer underneath as edit commands, and Escape didn't close it.
- Gated ImGuiInputHandler's unconditional fprintf/fflush diagnostics behind
  an IMGUI_IH_DEBUG macro, matching QtInputHandler's existing convention.

Syntax highlighting:
- Go/Rust/SQL highlighters weren't stateful, so multi-line /* */ comments
  mis-highlighted as code starting on the second line.
- Python's triple-quoted-string handler didn't re-scan the remainder of a
  line after a string closed mid-line, missing a same-line reopen.
- HighlighterEngine's state_last_contig_ field was declared but unused,
  forcing an O(n) scan of state_cache_ on every stateful lookup; wired it up
  as a real fast path and fixed InvalidateFrom() to keep it in sync.
- kge's :syntax off was silently undone the next frame because
  apply_syntax_to_buffer() re-applied the config default unconditionally
  every frame; added a user-override flag mirroring the existing
  edit-mode-detection guard.

Added regression tests for all of the above (test_undo.cc, test_kkeymap.cc,
test_command_semantics.cc, test_search_replace_flow.cc, and a new
test_syntax_highlighting.cc). Full suite (163 tests) passes, including under
AddressSanitizer.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

ImGuiInputHandler.cc

@@ -9,6 +9,19 @@
 #include "KKeymap.h"
 #include "Editor.h"
 
+// Verbose k-prefix suffix logging for debugging macOS/SDL key translation
+// issues. Default to off; enable by defining IMGUI_IH_DEBUG=1 at compile
+// time. Mirrors QtInputHandler.cc's QT_IH_DEBUG gate.
+#ifndef IMGUI_IH_DEBUG
+#define IMGUI_IH_DEBUG 0
+#endif
+
+#if IMGUI_IH_DEBUG
+#define IH_LOGF(...) do { std::fprintf(stderr, __VA_ARGS__); std::fflush(stderr); } while (0)
+#else
+#define IH_LOGF(...) ((void) 0)
+#endif
+
 
 static bool
 map_key(const SDL_Keycode key,
@@ -182,18 +195,19 @@ map_key(const SDL_Keycode key,
 			k_ctrl_pending             = false;
 			CommandId id;
 			bool mapped = KLookupKCommand(ascii_key, pass_ctrl, id);
+#if IMGUI_IH_DEBUG
 			// Diagnostics for u/U
 			if (lower == 'u') {
 				char disp = (ascii_key >= 0x20 && ascii_key <= 0x7e)
 					            ? static_cast<char>(ascii_key)
 					            : '?';
-				std::fprintf(stderr,
-				             "[kge] k-prefix suffix: sym=%d mods=0x%x ascii=%d '%c' ctrl2=%d pass_ctrl=%d mapped=%d id=%d\n",
-				             static_cast<int>(key), static_cast<unsigned int>(mod), ascii_key, disp,
-				             ctrl2 ? 1 : 0, pass_ctrl ? 1 : 0, mapped ? 1 : 0,
-				             mapped ? static_cast<int>(id) : -1);
-				std::fflush(stderr);
+				IH_LOGF(
+					"[kge] k-prefix suffix: sym=%d mods=0x%x ascii=%d '%c' ctrl2=%d pass_ctrl=%d mapped=%d id=%d\n",
+					static_cast<int>(key), static_cast<unsigned int>(mod), ascii_key, disp,
+					ctrl2 ? 1 : 0, pass_ctrl ? 1 : 0, mapped ? 1 : 0,
+					mapped ? static_cast<int>(id) : -1);
 			}
+#endif
 			if (mapped) {
 				out = {true, id, "", 0};
 				if (ed)
@@ -524,15 +538,17 @@ ImGuiInputHandler::ProcessSDLEvent(const SDL_Event &e)
 					bool pass_ctrl  = k_ctrl_pending_;
 					k_ctrl_pending_ = false;
 					bool mapped     = KLookupKCommand(ascii_key, pass_ctrl, id);
+#if IMGUI_IH_DEBUG
 					// Diagnostics: log any k-prefix TEXTINPUT suffix mapping
-					char disp = (ascii_key >= 0x20 && ascii_key <= 0x7e)
-						            ? static_cast<char>(ascii_key)
-						            : '?';
-					std::fprintf(stderr,
-					             "[kge] k-prefix TEXTINPUT suffix: ascii=%d '%c' mapped=%d id=%d\n",
-					             ascii_key, disp, mapped ? 1 : 0,
-					             mapped ? static_cast<int>(id) : -1);
-					std::fflush(stderr);
+					{
+						char disp = (ascii_key >= 0x20 && ascii_key <= 0x7e)
+							            ? static_cast<char>(ascii_key)
+							            : '?';
+						IH_LOGF("[kge] k-prefix TEXTINPUT suffix: ascii=%d '%c' mapped=%d id=%d\n",
+						       ascii_key, disp, mapped ? 1 : 0,
+						       mapped ? static_cast<int>(id) : -1);
+					}
+#endif
 					if (mapped) {
 						mi = {true, id, "", 0};
 						if (ed_)