vmap

JAX visitors for vmap expressions.

Visitors: _Placeholder, Vmap

Lowers a Vmap node to a jax.vmap over its body, mapping each batch argument to a _Placeholder that the body reads back through params. Batch data is resolved per source: constants are baked into the closure at lowering time, while Parameter/State/Control batches are gathered from params/x/u at runtime so they track SCP updates. Because the mapped body is a nested sub-trace that re-reads the same captured (x, u, node), the visitor pauses the value memo around the vmap so no cached tracer leaks across the trace boundary.