Python: add new shared-CFG-backed control flow graph facade (Cfg)

Adds the public facade on top of the AstNodeImpl adapter from the
previous commit. Re-exposes the same API surface as
semmle/python/Flow.qll (ControlFlowNode, CallNode, BasicBlock,
NameNode, DefinitionNode, CompareNode, ...), backed by the shared
codeql.controlflow.ControlFlowGraph library.

- semmle.python.controlflow.internal.Cfg — public facade.
- ControlFlow/store-load/* — basic store/load coverage via the facade.

The new CFG library is added additively: it has zero callers in lib/
and src/, and the legacy CFG in semmle/python/Flow.qll remains the
default. Dataflow, SSA, and production query migration land in
follow-up PRs.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: yoff

python/ql/lib/change-notes/2026-05-19-add-shared-cfg.md

@@ -0,0 +1,4 @@
+---
+category: minorAnalysis
+---
+* A new Python control flow graph implementation has been added under `semmle.python.controlflow.internal.Cfg` (backed by `AstNodeImpl.qll`), built on the shared `codeql.controlflow.ControlFlowGraph` library. It is not yet used by the dataflow library or any production query; the legacy CFG in `semmle/python/Flow.qll` remains the default. The new library is exposed for tests and for upcoming migrations.