audit: #317 cycle close — residues homed, gang-refusal claim pinned

Architect (fable gate): drift_found, minor, no contract violation. What
holds: invariant 9 / engine-store split (Op::Use resolves only through the
injected closure; the label sidecar is the C24 §Enforcement-shift-permitted
store-layer discovery, the splice inlines so the marketplace Forbids clause
is untouched); the C1 twin oracle; the open-pattern split consistent on both
cadences across C24, guide, module docs, and the flipped pin.

Drift resolutions:
- [medium] #317 minute 4621 over-claimed "gangable" -> correcting comment
  4629 posted; all prose surfaces already state "sweepable, not gangable".
- [medium] four accepted residues had no tracker home -> collective issue
  #339 (instance-param ganging, gang-knob binding at splice, open-run
  refusal rendering, --unwired on use-bearing documents) per the
  consolidation practice.
- [low] the documented "gang refuses an instance member path" claim was
  unpinned -> pinned here: gang_of_a_spliced_instance_member_path_refuses
  (this commit's one code change).
- [low] --unwired on use-bearing documents -> #339 item 4.

Regression: aura-bench report-only, all five surfaces fingerprint OK —
engine_throughput -0.2% bars_per_s, ingest +0.0%, campaign_sweep +0.1% cpu /
+0.5% rss / -0.3% wall, campaign_heavy +1.2% cpu / -8.8% rss / -0.9% wall,
cli_fixed_cost +1.2% help / -1.8% run. No baseline moved, nothing to ratify.

Erratum (batch-2 report): the harvest-sweep fieldtest spec consumed this
cycle; removed with the plan/spec sweep at this close.

refs #317

crates/aura-engine/src/construction.rs

@@ -1493,6 +1493,32 @@ mod tests {
         );
     }
 
+    /// Documented residue pin (#317 audit): a `gang` op cannot fuse a spliced
+    /// instance's member param — a gang fuses a PRIMITIVE's raw `(name, pos)`
+    /// slot, and an instance's params are nested/path-qualified. The refusal
+    /// is the ordinary no-such-open-param shape on the instance node, so the
+    /// authoring-guide/C24 claim "the gang op refuses a composite instance's
+    /// member path" stays an observed fact, not prose.
+    #[test]
+    fn gang_of_a_spliced_instance_member_path_refuses() {
+        let subgraph = |_: &str| Some(use_fixture());
+        let mut s = GraphSession::new("g", &std_vocabulary, &subgraph);
+        s.apply(Op::Use { ref_id: "fixture".into(), name: Some("gate".into()), bind: vec![] })
+            .unwrap();
+        s.apply(Op::Add { type_id: "SMA".into(), as_name: Some("solo".into()), bind: vec![] })
+            .unwrap();
+        assert_eq!(
+            s.apply(Op::Gang {
+                as_name: "fused".into(),
+                into: vec!["gate.sma.length".into(), "solo.length".into()],
+            }),
+            Err(OpError::BadParam {
+                node: "gate".into(),
+                err: BindOpError::UnknownParam("sma.length".into()),
+            })
+        );
+    }
+
     /// DAG invariant (domain invariant 5 / C9), through an instance: a
     /// `connect` closing a cycle through a spliced instance is rejected
     /// eagerly, the instance treated as one opaque node (#317) — the same