fix(cypher): preserve node properties through WITH aggregation

[KerseyFabrications]

A node group variable carried through a WITH aggregation
(e.g. WITH g, count(*) AS c RETURN g.file_path) returned blank for every
property except its name: the carried virtual binding held only the group
key (the node's name) and lacked a store handle, so node_prop() could
neither read other fields nor compute degrees.
Fix: capture the node id of a bare node group-var in with_agg_find_or_create
and tag the carried virtual binding with it; in node_prop(), when such a stub
(id set, string fields unpopulated) is asked for a missing property, re-fetch
the full node via cbm_store_find_node_by_id and project it. Also propagate
the store onto virtual bindings so node_prop can re-fetch and compute
degrees. The stub gate is heuristic but never yields a wrong value — worst
case is one redundant indexed lookup. Adds regression test
cypher_exec_with_node_groupvar_prop.

Signed-off-by: Kris Kersey kris@kerseyfabrications.com

[DeusData]

Thanks @KerseyFabrications — preserving node properties through WITH aggregation is a real fix and the regression test is good. One correctness issue before merge: using empty-string as the stub sentinel (if (str && str[0])) means a node legitimately stored with an empty string field (e.g. an empty file_path on a synthetic node) falls through to a re-fetch on every property access. Please use an explicit stub flag (or carry the node id through the aggregation path) instead of repurposing the empty-string sentinel. 🙏