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
Reorder recursive byte-stream consumers so the consumed input is inspected
before loop-control arguments can drive evaluation. Previously, partially
applying `readBytes` to a known count, such as `readBytes 2`, allowed the
evaluator to specialize the recursive worker using known counter values
while the byte stream was still abstract. This caused symbolic recursion
over unknown input and produced an enormous normal form.
The recursive worker now takes the byte stream first and immediately
case-analyzes it. As a result, partial application blocks at the input
boundary instead of unrolling the counter loop.
This preserves the fully-applied behavior of `readBytes`, while making partial
application such as `readBytes 2` normalize safely.
