Only freeHashed is written to (via freeHashedNode); the internal and
stem lists are declared, consumed in alloc paths, and copied in
NodeStore.Copy, but no callsite ever appends to them. Under current
semantics (no delete, stem-split keeps the old stem deeper in the
tree) there is no path that would free an internal or stem slot, so
the recycle branch was dead code. Drop it to avoid misleading future
contributors; the infrastructure is easy to restore if a delete path
is ever added.