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.
Brummel
a8bd52eaff