Four crate-gated tasks for spec 0016: aura-core (LeafFactory + Scalar accessors),
aura-std (factory() on the 7 nodes), aura-engine (value-empty Leaf, build-then-wire
compile_with_params/bootstrap_with_params, kind+arity errors, vestigial schema
removal, fixtures + injection tests re-expressed), aura-cli (param-generic render,
sample + goldens, swap moved to the compiled view). Compile gates are crate-scoped
so each task's gate is satisfiable with all its callers threaded in-task.
refs #31
