test: normalizing passes need an idempotence oracle #689

Problem

#67 plans passes that rewrite the document (the const-union normalization, usage flags, filtering). A normalization that changes its own output on a second run is a defect the current oracles can't see.

Proposal

When the pass framework lands, add an oracle that runs each transforming pass twice and requires the second run to change nothing, next to determinism and order-invariance.