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>

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.

python/ql/test/library-tests/ControlFlow/store-load/StoreLoadTest.ql

@@ -0,0 +1,41 @@
+/**
+ * Inline-expectations test for the store/load/delete/parameter
+ * classification predicates on the new-CFG facade.
+ *
+ * Each tag fires when the corresponding predicate (`isLoad`,
+ * `isStore`, `isDelete`, `isParameter`, `isAugLoad`, `isAugStore`)
+ * holds on the canonical CFG node wrapping a `Py::Name` with the
+ * given identifier. Subscript and attribute stores are not covered
+ * by these tags — only the `Name`-typed targets/loads they involve.
+ */
+
+import python
+import semmle.python.controlflow.internal.Cfg as Cfg
+import utils.test.InlineExpectationsTest
+
+module StoreLoadTest implements TestSig {
+  string getARelevantTag() { result = ["load", "store", "delete", "param", "augload", "augstore"] }
+
+  predicate hasActualResult(Location location, string element, string tag, string value) {
+    exists(Cfg::NameNode n |
+      location = n.getLocation() and
+      element = n.toString() and
+      value = n.getId() and
+      (
+        n.isLoad() and not n.isAugLoad() and tag = "load"
+        or
+        n.isStore() and not n.isAugStore() and tag = "store"
+        or
+        n.isDelete() and tag = "delete"
+        or
+        n.isParameter() and tag = "param"
+        or
+        n.isAugLoad() and tag = "augload"
+        or
+        n.isAugStore() and tag = "augstore"
+      )
+    )
+  }
+}
+
+import MakeTest<StoreLoadTest>

python/ql/test/library-tests/ControlFlow/store-load/test.py

@@ -0,0 +1,56 @@
+# Store/load/delete/parameter classification on the new-CFG facade.
+#
+# Each annotated location carries the (sorted, deduplicated) set of
+# kinds the CFG facade reports there. Comparing against the legacy
+# 'semmle.python.Flow' classification is done by the comparison query
+# 'StoreLoadParity.ql' — annotations here are only the positive
+# assertions for the new facade.
+#
+# Tags:
+#   load=<id>       -- isLoad() fires on the Name
+#   store=<id>      -- isStore() fires
+#   delete=<id>     -- isDelete() fires
+#   param=<id>      -- isParameter() fires
+#   augload=<id>    -- isAugLoad() fires (the LHS of x += ... when read)
+#   augstore=<id>   -- isAugStore() fires (the LHS of x += ... when written)
+
+
+# --- plain load / store / delete ---
+
+x = 1  # $ store=x
+y = x + 1  # $ store=y load=x
+print(y)  # $ load=print load=y
+del x  # $ delete=x
+
+
+# --- function definitions (parameters) ---
+
+def f(a, b=2, *args, c, **kwargs):  # $ store=f param=a param=b param=args param=c param=kwargs
+    return a + b + c  # $ load=a load=b load=c
+
+
+# --- augmented assignment splits one Name into load + store halves ---
+
+def aug():  # $ store=aug
+    n = 0  # $ store=n
+    n += 1  # $ augload=n augstore=n
+    return n  # $ load=n
+
+
+# --- subscript / attribute stores ---
+
+class C:  # $ store=C
+    pass
+
+
+def stores(obj, container, idx):  # $ store=stores param=obj param=container param=idx
+    obj.attr = 1  # $ load=obj
+    container[idx] = 2  # $ load=container load=idx
+    return obj  # $ load=obj
+
+
+# --- tuple unpacking ---
+
+def unpack(pair):  # $ store=unpack param=pair
+    a, b = pair  # $ store=a store=b load=pair
+    return a + b  # $ load=a load=b