C#: Add change note.

Author: aschackmull

csharp/ql/lib/change-notes/2026-04-13-cfg.md

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+---
+category: breaking
+---
+* The C# control flow graph (CFG) implementation has been completely
+  rewritten. 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.
+  Additionally, the following breaking changes have been made:
+  - `ControlFlow::Node` has been renamed to `ControlFlowNode`.
+  - `ControlFlow::Nodes` has been renamed to `ControlFlowNodes`.
+  - `BasicBlock.getCallable` has been renamed to `BasicBlock.getEnclosingCallable`.
+  - `BasicBlocks.qll` has been deleted.
+  - `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
+    `ControlFlowNode.asExpr()` or `ControlFlowNode.asStmt()` or
+    `ControlFlowElement.getControlFlowNode()` instead.