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>

UndoSystem.cc

@@ -37,7 +37,7 @@ UndoSystem::Begin(UndoType type)
 
 	// Some operations should always be standalone undo steps.
 	const bool always_standalone = (type == UndoType::Newline || type == UndoType::DeleteRow || type ==
-	                                UndoType::InsertRow);
+	                                UndoType::InsertRow || type == UndoType::JoinLines);
 	if (always_standalone) {
 		commit();
 	}
@@ -77,6 +77,7 @@ UndoSystem::Begin(UndoType type)
 			case UndoType::Newline:
 			case UndoType::DeleteRow:
 			case UndoType::InsertRow:
+			case UndoType::JoinLines:
 				break;
 			}
 		}
@@ -325,6 +326,16 @@ UndoSystem::apply(const UndoNode *node, int direction)
 			buf_->SetCursor(0, static_cast<std::size_t>(node->row));
 		}
 		break;
+	case UndoType::JoinLines:
+		// Mirror image of Newline: forward removes a newline, backward restores it.
+		if (direction > 0) {
+			buf_->join_lines(node->row);
+			buf_->SetCursor(static_cast<std::size_t>(node->col), static_cast<std::size_t>(node->row));
+		} else {
+			buf_->split_line(node->row, node->col);
+			buf_->SetCursor(0, static_cast<std::size_t>(node->row + 1));
+		}
+		break;
 	}
 }
 
@@ -424,6 +435,8 @@ UndoSystem::type_str(UndoType t)
 		return "DeleteRow";
 	case UndoType::InsertRow:
 		return "InsertRow";
+	case UndoType::JoinLines:
+		return "JoinLines";
 	}
 	return "?";
 }