v2 Stage 4: L4 interprocedural SDG + Scalpel oracle

[rahlk]

Part of #69.

Stage 4 — L4 interprocedural SDG + Scalpel oracle

Branch: feat/v2-stage4-l4

Deliverables:

• dataflow/scalpel_oracle.py — ScalpelAliasOracle implementing the may_alias(p1, p2) contract (per the Stage 0 record); oracle selection: Scalpel primary, TypeBasedAliasOracle fallback (logged when it falls back).
• Emit synthetic param vertices into body (@formal_in/out, …/actual_in/out); summary edges on the callable; param_in/param_out at application scope; semantic DDG (prov:["points-to"]) as the delta the oracle adds beyond L3.
• CLI: -a max 3→4; --graphs sdg requires -a 4.
• Neo4j: param vertices ride PyCFGNode; PY_PARAM_IN/OUT, PY_SUMMARY.
• pyproject.toml: add [project.optional-dependencies] scalpel.
• Migrate test/test_dataflow_sdg.py, test/test_dataflow_emission.py.

Files: dataflow/{scalpel_oracle,alias,builder,sdg,summaries}.py, __main__.py, neo4j/project.py, pyproject.toml, SDG tests.

Gate: no dangling SDG endpoints; PARAM_IN/OUT arity matches; a summary edge exists for a known transitive flow in the fixture; full superset L1 ⊆ … ⊆ L4; L4 gate runs against the Scalpel oracle (monotonic widening only).

Expand into full bite-sized tasks with superpowers:writing-plans at stage start.

[rahlk]

Stage-4 note (from Stage 3 Task 3.8): the L3 conformance gate asserts every ddg edge has prov == ["ssa"] under max_level >= 3. When L4 lands the alias-derived (prov: ["points-to"]) ddg edges, relax this in test/conftest_v2.py::assert_conformant (gate the ssa-only check to exactly level 3, or widen the allowed prov set at level 4). Also: emit_l3_body hardcodes prov=["ssa"] — the L4 emission must add the points-to edges as the delta.