refactor(aura-core): own FieldSpec.name to drop the derive_signature heap leak

FieldSpec.name was &'static str, forcing derive_signature to Box::leak
every re-exported composite OutField name. Because compile_with_params
reruns derive_signature per sweep point, an N-point sweep over an
M-output-field composite leaked N×M boundary names unbounded — a scaling
wall the moment the World/sweep layer compiles families per point.

Pull FieldSpec.name to String, mirroring the already-owned PortSpec.name
and ParamSpec.name: a composite re-exports interior fields under runtime
boundary names, so ownership is the correct model, not a workaround.
FieldSpec loses Copy; the only fallout is one borrow at the render site.
leak_name/Box::leak deleted.

Behaviour-preserving: signature contents unchanged, all 231 tests green.

crates/aura-core/src/node.rs

@@ -33,8 +33,9 @@ pub enum Firing {
 /// window = its `length`). The `name` is a **non-load-bearing** debug symbol (C23):
 /// identity is the positional slot, the name is for tracing / graph rendering
 /// (#13), never read by bootstrap or the run loop (which wire by index). A
-/// `String`, not `&'static str` like `FieldSpec.name`: a variadic node generates
-/// per-slot names (`term[0]`), exactly as `ParamSpec.name` does (`weights[0]`).
+/// `String` (as `FieldSpec.name` and `ParamSpec.name` are): a variadic node
+/// generates per-slot names (`term[0]`), needing ownership, exactly as
+/// `ParamSpec.name` does (`weights[0]`).
 #[derive(Clone, Debug, PartialEq, Eq)]
 pub struct PortSpec {
     pub kind: ScalarKind,
@@ -45,10 +46,13 @@ pub struct PortSpec {
 /// One declared output column of a node's record: its name (metadata for sinks /
 /// the playground, C18) and its scalar kind. The position of a `FieldSpec` in
 /// `NodeSchema.output` is what an `Edge` binds (`Edge::from_field`); the name is
-/// not load-bearing for wiring.
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+/// not load-bearing for wiring. The `name` is a `String` (not `Copy`): a composite
+/// re-exports an interior field under a runtime boundary name (`OutField.name`,
+/// derived in `derive_signature`), so ownership is required — the same reason
+/// `PortSpec.name` and `ParamSpec.name` are owned.
+#[derive(Clone, Debug, PartialEq, Eq)]
 pub struct FieldSpec {
-    pub name: &'static str,
+    pub name: String,
     pub kind: ScalarKind,
 }
 
@@ -56,8 +60,8 @@ pub struct FieldSpec {
 /// kind. The name is a **non-load-bearing** debug symbol (path-qualified at
 /// aggregation, as `FieldSpec.name` already is); a param's identity is its
 /// positional slot in the blueprint's aggregated param-space (C23 — by index, not
-/// by name). Unlike `FieldSpec.name` (`&'static str`), the name is a `String`: a
-/// vector knob carries a runtime index (`weights[0]`) and aggregation prefixes the
+/// by name). The name is a `String` (as `FieldSpec.name` and `PortSpec.name` are):
+/// a vector knob carries a runtime index (`weights[0]`) and aggregation prefixes the
 /// composite path (`strategy.weights[0]`). Permitted kinds: `I64`/`F64`/`Bool`;
 /// `Timestamp` is a structural axis (C20), never a numeric knob.
 #[derive(Clone, Debug, PartialEq, Eq)]