J_PARAM_IN/J_PARAM_OUT declare a var property the projection never writes #249

Problem

V2SchemaCatalog declares a var property on both interprocedural SDG relationship types:

r.add(rel("J_PARAM_IN",  body, body, new P().put("var", "string").done()));
r.add(rel("J_PARAM_OUT", body, body, new P().put("var", "string").done()));

and V2GraphProjector emits both with RowBuilder.props() — no properties at all. So the
declaration promises a property no edge carries. Same defect as codeanalyzer-python#195, found
while fixing that one; verified against 3.1.1's own dataflow-test fixture.

The consequence is not cosmetic. A Cypher predicate written against the declaration lands in
three-valued logic on real data: all(r IN relationships(p) WHERE r.var <> $v) evaluates to null
on any path crossing a call boundary, so the path is silently excluded and the result is
indistinguishable from a genuine "no flow".

The variable is known at emit time and is already computed: SdgVertices.addActualsAndEdges binds
each actual_in:i to callee.getParameters().get(formalIndex), and each actual_out to the
callee's @formal_out. JIdEdge simply has nowhere to carry it.

Scope boundary

JIdEdge, SdgVertices, V2GraphProjector and their tests. Python took the same direction
(write it, in both projections) in codeanalyzer-python#196. Not the summary edges, which share
JIdEdge but bind no formal. Not the body-node / parameter id gap, which is a separate child of
codellm-devkit/.github#56.

Goals

• JIdEdge.var, carried into analysis.json on param_in / param_out
• SdgVertices sets it: the bound parameter's name on param_in, $ret on param_out
• V2GraphProjector writes it on J_PARAM_IN / J_PARAM_OUT
• A test asserts the property is on every emitted edge of those types, not some
• summary edges keep the bare shape (var absent, never null-serialized)

Caveats and known risks

• New property downstream consumers may key on; graphs emitted by 3.1.1 and earlier will not have
  it, so a consumer predicate still has to tolerate its absence for a generation.
• JIdEdge is shared with summary. Gson is configured without serializeNulls, so an unset
  var is omitted rather than written as null — verify that holds for the summary lists.
• The divergence is invisible to any test that counts edges or checks endpoints. Only a test that
  reads properties off emitted edges can see it.

Definition of done

• Catalog and projector agree for both relationship types, asserted against emitted rows rather
  than by reading the two files.
• Every param_in / param_out edge in the dataflow-test L4 output carries a non-empty var.
• Full ./gradlew test green.