feat(aura-engine): Op::Use splices a registered blueprint; open patterns build

The construction session gains the use op: Op::Use { ref_id, name, bind }
resolves through a second injected closure (subgraph: &dyn Fn(&str) ->
Option<Composite>, mirroring vocab's closed-lookup posture — the engine never
touches the store) and pushes the fetched composite as BlueprintNode::Composite,
renamed to the instance identifier (names are identity-blind debug symbols),
with path-qualified binds applied at splice time. Port resolution, kind checks,
the eager cycle gate, and the holistic finish gates all walk the existing
Composite arm — the session storage already carried it; the
"sessions only ever add primitives" unreachable! is retired by construction.

Open patterns (ratified mid-cycle, #317 comment 4627): finish() drops the
root-role gate; compile/bootstrap keep it. An op-script's explicit input roles
are a pattern's formal parameters — buildable, registerable, splicable; running
one standalone still refuses at bootstrap. The pinned finish-refusal test flips
to assert the new split (finish OK, compile refuses). No shipped fixture used
the input op; no corpus behaviour moves.

Gang integrity at the use seam (review finding): binding a ganged member's raw
path refused via the new BindOpError::AlreadyGanged { param, gang } (aura-core)
instead of silently de-fusing the gang — membership keyed on the same
(node, original_pos) coordinate collect_params/check_gangs use, per nesting
level. The gang's public knob stays unbindable at the seam (recorded residue).

Correctness oracle: use_op_splices_byte_identical_to_the_rust_built_twin pins
blueprint_to_json byte-equality against the GraphBuilder twin AND a
bit-identical recorded run (C1). Seven further use_op_* tests cover the fault
surfaces; open_input_pattern_finishes_registers_shaped_and_splices covers the
open-pattern round trip build -> serialize -> reload -> splice -> run.

No blueprint-format change: the serialized form already nests composites
recursively; the golden byte pin stays untouched.

refs #317

crates/aura-engine/tests/construction_e2e.rs

@@ -58,7 +58,7 @@ fn signal_ops() -> Vec<Op> {
 #[test]
 fn replayed_construction_compiles_identically_to_graphbuilder() {
     // (a) op-script replay through the public construction surface.
-    let replay_flat = replay("root", signal_ops(), &std_vocabulary)
+    let replay_flat = replay("root", signal_ops(), &std_vocabulary, &|_: &str| None)
         .expect("op-script resolves")
         .compile_with_params(&params())
         .expect("replay compiles");
@@ -103,7 +103,7 @@ fn replay_attributes_eager_fault_to_its_op_index() {
     ];
     // `.err().unwrap()` (not `unwrap_err`): the Ok arm `Composite` holds build
     // closures and is not `Debug`, which `unwrap_err`'s bound would require.
-    let err = replay("g", ops, &std_vocabulary).err().unwrap();
+    let err = replay("g", ops, &std_vocabulary, &|_: &str| None).err().unwrap();
     assert_eq!(err, (2, OpError::UnknownNodeType("Nope".into())));
 }
 
@@ -128,7 +128,7 @@ fn replay_attributes_holistic_fault_to_finalize_step() {
         // sub.rhs deliberately left unwired -> only finalize can decide totality.
     ];
     let finalize_index = ops.len(); // 6 — the implicit finalize step
-    let err = replay("g", ops, &std_vocabulary).err().unwrap();
+    let err = replay("g", ops, &std_vocabulary, &|_: &str| None).err().unwrap();
     assert!(matches!(&err, (i, OpError::UnconnectedPort { .. }) if *i == finalize_index),
         "the holistic fault is still attributed to the finalize step, got {err:?}");
 }