feat(neo4j)!: schema v2 graph projection; v2 becomes the default schema (3.0.0)

[rahlk]

Closes #198. Implements the base relabel + L3 overlay of #182 (L1/L2 families + J_CFG_NEXT/J_CDG/J_DDG); the L4 SDG overlay (J_PARAM_*/J_SUMMARY, declared-only in the catalog here) remains for after L4. Implements docs/design/specs/2026-08-27-v2-neo4j-projection.md.

What

• V2GraphProjector — pure (Analysis, appName) → GraphRows, projecting the v2 model onto the 2.0.0 graph vocabulary (python-parity: JSymbol merge label over JType/JCallable/JExternal; JBodyNode keyed on the global ordinal replaces call-site nodes; parameters_json/docstring flattening; J_CFG_NEXT/J_CDG/J_DDG overlay edges with the _k MERGE discriminant).
• V2SchemaCatalog — declarative 2.0.0 contract, derived uniqueness constraints; --emit schema now always emits it; repo-root schema.neo4j.json regenerated.
• RowBuilder/GraphRows/writers gain the optional edge _k discriminant (a conditional's true/false CFG pair, one DDG dependence per (var, prov)); CypherWriter/BoltWriter MERGE on {_k: row.k} for keyed groups.
• Cross-generation replacement: the wipe/prune traversal spans v1 and v2 containment (J_HAS_UNIT|J_HAS_MODULE + union DESCENDANTS incl. J_HAS_BODY_NODE) and sweeps orphaned degree-0 symbols (v1's import-materialized bodyless JType stubs). Bolt diff/prune handles both generations and scopes pruning to the app anchor.

Breaking (3.0.0)

• --schema defaults to v2 for json and neo4j; --schema v1 selects the legacy shapes (v1 node/edge cypher statements unchanged; the shared DDL/wipe preamble gained the v2 union, required for replacement in both directions).
• --emit neo4j at v2 is always full depth (forces L3 + external symbols); -a/--graph-field-depth alongside it exit non-zero (keystone depth rule, python Fix Issue 113: call graph is missing edges to implementations of interfaces. Merge to v1.X.X. #119 parity).
• --emit schema always emits the 2.0.0 catalog.

python-sdk is shielded by its exact codeanalyzer-java == "2.4.1" pin; its compat PR (bump + explicit --schema v1) is separate work per the spec's release plan.

Verification

• Full suite green except CodeAnalyzerIntegrationTest (pre-existing Testcontainers/Docker env failure, identical on main).
• New V2Neo4jSchemaConformanceTest: every projected label/rel/prop declared; body-node convergence pinned (no JCompilationUnit/JCallSite/JParameter/JComment rows); keyed overlay edges; wipe coverage.
• E2E against Neo4j 5 (this repo as input, 30223 nodes / 158704 rels): cypher-shell loads clean; v1 load → v2 load leaves 0 v1 leftovers (including the orphaned import stubs); reloading is idempotent (counts unchanged); graph counts equal the same run's analysis.json exact-set for modules (156), callables (1199), body nodes (18265), cfg (8473), cdg (3031), ddg (5465); J_CALLS/JExternal counts equal the emitting run's rows exactly (cross-run RTA jitter of ±36 edges exists between separate analyses — pre-existing WALA behavior, not projection loss).
• Traversal sanity: JApplication→J_HAS_MODULE→J_DECLARES→J_HAS_METHOD→J_HAS_BODY_NODE→J_RESOLVES_TO returns resolved callees; CFG edge kinds distribute as expected (fallthrough/true/false/return).