Add RelaxNG.validate(streaming:), driving the Brzozowski derivative directly
from the parse-event stream (the libxml2 xmlTextReader model). The derivative
is already incremental: the after-threaded pattern chain is the validation
stack, so only the residual pattern and a light per-element frame (for the
childrenDeriv ignorable-whitespace rule) are retained, never the tree. A
differential test asserts streaming agrees with the trusted tree engine across
valid, invalid, whitespace, and mixed-content documents. XSD streaming remains.
Part of #107, #105
