Enforced pipeline idempotency: stitch(stitch(x)) == stitch(x) now holds
across the full option matrix. Append-side gates refuse an append a later
handler would swallow into a region, and a bounded fixpoint loop in
run_pipeline settles the cross-handler shapes no single gate can.

The idempotency sweep fuzz_idempotent_all_option_combinations is unignored
and passes at 300k cases. No public API changes.