Schema v2 migration + four analysis levels (epic) #69

Epic: migrate codeanalyzer-python to canonical schema v2 + four analysis levels

Migrate the analyzer from the v1 payload (PyApplication + a separate program_graphs section) to the CLDK canonical schema v2 — one additive Code Property Graph tree — exposed as four gated analysis levels (-a 1|2|3|4), in both the analysis.json and Neo4j projections. This is a breaking major release (the codeanalyzer-backend "existing analyzer → schema v2" path). It supersedes the L3 approach in #67: dataflow moves inline into the tree, and the interprocedural SDG moves from -a 3 to -a 4.

Target shape

application → symbol_table{module} → types/functions → callables{ body{}, cfg[], cdg[], ddg[], summary[] }
call_graph[], param_in[], param_out[]   — cross-function edges, at application scope

• can://python/<app>/<file>/<type>/<callable-sig> durable ids (≥ callable); <callable-id>@<line>:<col> / @tag ordinal ids (< callable).
• source stored once per module + byte-offset spans (every node's text slices off it); the per-callable code field is dropped.
• Payload envelope: schema_version: "2.0.0", language, max_level, k_limit.

Levels

--graphs sdg requires -a 4 (requesting it at -a 3 is a flag error).

Key decisions

• Reuse the existing dataflow kernels (cfg/dominance/defuse/sdg/summaries/alias); a new identity-mapping layer translates internal (signature, node_id) keys → can:// ids at emit time, which keeps the JSON and Neo4j projections in lockstep.
• Scalpel (python-scalpel) is the primary L4 points-to oracle behind the frozen may_alias interface, with TypeBasedAliasOracle as an automatic fallback.
• Analyzer-only scope. The python-sdk Pydantic revision is a separate follow-up (via cldk-sdk-frontend). Taint stays deferred to the SDK; the backward slicer stays internal as the L3/L4 validation gate (no product "queryable slicer" framing, no taint_flows section).
• Neo4j is a re-identify + one prov property + source/span parity + MAJOR bump (schema.neo4j.json → 2.0.0); the near-identity projection structure is unchanged. Always full-depth.

Invariants / gates

• Additive superset: analysis.json(-a 1) ⊆ … ⊆ analysis.json(-a 4), modulo the callee: null→id refinement and the L3→L4 DDG ssa→points-to widening.
• Two-projection agreement: JSON node/edge ids == Neo4j merge-key values.
• Per-level gates (validated per stage): L1 canonical-schema conformance; L2 no dangling call-graph endpoints; L3 intraprocedural backward slice == hand-computed set; L4 no dangling SDG endpoints + PARAM arity + a summary for a known transitive flow.

Stages

Each stage is an independently shippable PR-unit with its own gate. Child issues:

• v2 Stage 0: Scalpel / toolchain verification spike #70 — Stage 0: Scalpel / toolchain verification spike
• v2 Stage 1: schema foundation + L1 #71 — Stage 1: schema foundation + L1
• v2 Stage 2: L2 (callee backfill + call_graph re-identity) #72 — Stage 2: L2 (callee backfill + call_graph re-identity)
• v2 Stage 3: L3 intraprocedural CFG/CDG/DDG (syntactic) #73 — Stage 3: L3 intraprocedural CFG/CDG/DDG (syntactic)
• v2 Stage 4: L4 interprocedural SDG + Scalpel oracle #74 — Stage 4: L4 interprocedural SDG + Scalpel oracle
• v2 Stage 5: docs, versioning, hand-off #75 — Stage 5: docs, versioning, hand-off