feat(dataflow): stage 1 — exceptional statement-level CFG per callable

Level-3 groundwork (#67): hand-built CFG from the stdlib ast with the
shared node/edge vocabulary, Python lowering rules (try/except/else/
finally, with, yield/await resume kinds, break/continue, synthetic
escape edge for infinite loops), dead-code pruning, and source-span-
ordered node ids (ENTRY=0, EXIT=last). Dataflow fixture project and
CFG gate tests included.

Author: rahlk

codeanalyzer/dataflow/__init__.py

@@ -0,0 +1,35 @@
+################################################################################
+# Copyright IBM Corporation 2025
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#       http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+################################################################################
+
+"""Level-3 native dataflow graphs: CFG, PDG (CDG + DDG), and the SDG.
+
+One pass per module, mirroring the construction ladder:
+
+- :mod:`cfg` — stage 1, exceptional statement-level CFG per callable;
+- :mod:`dominance` — stage 2, post-dominators and control dependence;
+- :mod:`access_paths` — stage 3a, the k-limited access-path variable model;
+- :mod:`defuse` — stage 3b, reaching definitions → DDG edges;
+- :mod:`alias` — stage 5, the type-based may-alias oracle (MVP stub);
+- :mod:`scc` — stage 5, Tarjan SCC condensation of the call graph;
+- :mod:`summaries` — stage 6, bottom-up formal-in → formal-out summaries;
+- :mod:`sdg` — stage 7, parameter nodes and CALL/PARAM_IN/PARAM_OUT/SUMMARY
+  edges;
+- :mod:`slicing` — stage 8, the two-phase context-sensitive backward slice;
+- :mod:`builder` — the orchestrator ``build_program_graphs`` wired into
+  ``Codeanalyzer.analyze`` at ``-a 3``.
+"""
+
+from codeanalyzer.dataflow.cfg import build_cfg  # noqa: F401