freeVars did not descend into TStem, TFork, or SList, so dependency analysis
under structural nodes and lists was silently missed.
toSKIDB's _other = _K `SApp` TLeaf fallback returned a constant leaf when the
binder occurred under a structural node, losing the abstraction entirely.
Replace with explicit lowering: BStem/BFork/BList are converted to application
form before SKI abstraction, and any other unsupported DB term errors explicitly
James Eversole
1d84bf7cfa