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>

ImGuiFrontend.cc

@@ -44,6 +44,12 @@ apply_syntax_to_buffer(Buffer *b, const GUIConfig &cfg)
 	if (!b->EditModeDetected() && !b->Filename().empty())
 		b->SetEditMode(DetectEditMode(b->Filename()));
 
+	// If the user explicitly set syntax state via a command (:syntax on/off,
+	// :set filetype=...), leave it alone - otherwise this runs every frame
+	// and silently undoes a manual ":syntax off" on the very next frame.
+	if (b->SyntaxUserOverride())
+		return;
+
 	// Writing mode disables syntax; otherwise follow the global config.
 	if (cfg.syntax && b->GetEditMode() != EditMode::Writing) {
 		b->SetSyntaxEnabled(true);
@@ -469,7 +475,19 @@ GUIFrontend::Step(Editor &ed, bool &running)
 
 		// Route input events to the correct window's input handler
 		if (target) {
-			target->input.ProcessSDLEvent(e);
+			Editor &tgt_ed = (target_idx == 0) ? ed : target->editor;
+			if (tgt_ed.FilePickerVisible()) {
+				// Modal: don't let keystrokes fall through as edit commands
+				// to the buffer underneath. Escape closes the picker;
+				// everything else (navigation, filtering, double-click) is
+				// handled by the picker's own ImGui widgets via
+				// ImGui_ImplSDL2_ProcessEvent above.
+				if (e.type == SDL_KEYDOWN && e.key.keysym.sym == SDLK_ESCAPE) {
+					tgt_ed.SetFilePickerVisible(false);
+				}
+			} else {
+				target->input.ProcessSDLEvent(e);
+			}
 		}
 	}