MR-925: experiment 1.3 \u2014 custom UserDefinedLogicalNode + ExecutionPlan e2e

- validation-prototypes/custom-operator/: NeighborExpand toy operator
  with paired ExtensionPlanner + custom QueryPlanner via
  SessionStateBuilder::with_query_planner
- writeup at .context/experiments/custom-operator.md: 5 probes
  (round-trip, EXPLAIN, predicate guard, composition with Filter +
  Aggregate, BaselineMetrics) \u2014 all pass; ~250 LoC integration
  footprint; no unsafe; no internal API access
- finding: \u00a75.3 is achievable on DF 52.5 as written; deltas are
  doc-shaped (predicate push-down opt-in, statistics requirement,
  Partitioning override)

validation-prototypes/Cargo.lock

@@ -1280,6 +1280,25 @@ dependencies = [
  "uuid",
 ]
 
+[[package]]
+name = "custom-operator"
+version = "0.0.0"
+dependencies = [
+ "anyhow",
+ "arrow",
+ "arrow-array",
+ "arrow-schema",
+ "async-trait",
+ "datafusion",
+ "datafusion-common",
+ "datafusion-execution",
+ "datafusion-expr",
+ "datafusion-physical-expr",
+ "datafusion-physical-plan",
+ "futures",
+ "tokio",
+]
+
 [[package]]
 name = "darling"
 version = "0.23.0"