scripts/fixpoint-native.sh verifies that the native backend is fully
self-hosting: stage-0 (QBE-compiled compiler) emits native assembly,
the linked stage-1 binary recompiles itself via --backend native to
produce stage-2, and stage-2 recompiles itself to produce stage-3.
A clean fixpoint means stage-2 and stage-3 assembly are identical.
Both fixpoints (QBE and native) are now required before every commit.
