Implement-time discovery: the new check (and the existing edge/role checks) compute
signature() on every interior node before the recursion validates that node's
interior. For a structurally-invalid composite (an out-of-range OutField or role
target), derive_signature indexed unguarded and PANICKED — exposed by
output_port_out_of_range_rejected the moment its covering root role forces
c.signature(). Added Task 1 Step 2b: make derive_signature and interior_slot_kind
bounds-total (a placeholder kind for an invalid index; the real fault is still
reported by validate_wiring's guarded OutputPortOutOfRange / BadInteriorIndex). Fix
verified empirically before re-dispatch. Not a design change to the check itself.
refs #65
Brummel
277f8714d4