test: cover sort and render ordering boundary cases

Added ERT tests for the deterministic-ordering guarantees: title sort folds nil and empty titles to the empty key (ordered first ascending); a nil (none) priority sorts last ascending and first descending; equal keys keep input order ascending and reverse it descending (Emacs sort is stable, then nreverse); and the render boundary sorts by priority after normalization even when a node omits the field. 384 tests green.

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