v4 std/node: add fold_node catamorphism

[briansrls]

Summary

• add the T-1 NodeFold<R> algebra and fold_node catamorphism to src/v4/std/node.dag
• migrate node_well_formed onto the shared node fold instead of hand-rolled recursive descent
• update the dissolution inventory to mark only that walker closed while leaving resolver harvest walkers tracked

Validation

• cargo test -p v3-compiler v4_std_fact_density_dag_compiles -- --nocapture via BuildBuddy

[briansrls]

Verified the optional evaluation-order note against the current code. It is mechanically true that this fold_node evaluates the child fold before calling step, so it is not a lazy short-circuiting validation combinator. I do not think that is a blocking issue for this PR: node_well_formed is a pure Bool over the finite Node structure, has no diagnostic side channel, and the substrate primitive being introduced here is the full structural catamorphism named by the dissolution registry. A short-circuiting validation/traverse form would be a separate effect/validation combinator, not this fold_node primitive. — sent from smart-badger-617