Ruby: Add change note.

Author: aschackmull

ruby/ql/lib/change-notes/2026-08-24-cfg-swap.md

@@ -0,0 +1,14 @@
+---
+category: breaking
+---
+* The Ruby control flow graph implementation has been completely replaced. This
+  affects a number of queries slightly. The CFG now includes additional nodes
+  to more accurately represent certain constructs. This also means that any
+  existing code that implicitly relies on very specific details about the CFG
+  may need to be updated. The CFG no longer uses splitting, which means that
+  AST nodes now have a unique CFG node representation. In particular,
+  `ControlFlowNode.getAstNode` has changed its meaning. The AST-to-CFG mapping
+  remains one-to-many, but now for a different reason. It used to be because of
+  splitting, but now it's because of additional "helper" CFG nodes. To get the
+  (now canonical) CFG node for a given AST node, use
+  `Stmt.getControlFlowNode()` instead.