None of the existing fixpoint scripts exercise the incremental warm-cache
rebuild path: fixpoint.sh uses --emit-ir (no cache), fixpoint-native.sh diffs
.s (no cache), fixpoint-native-internal.sh compiles a tiny program. So a
regression in the cached-interface import/round-trip (dropped field, wrong
vtable slot, duplicate global, missing init) passes all of them and only
surfaces later as a corrupt binary — exactly the class of bug just fixed.
scripts/fixpoint-warmcache.sh builds the whole compiler twice into the same
--unit-cache (clean then warm), asserts the warm rebuild is non-empty and not
drastically smaller than the clean build, and compiles+runs hello-world with
both stage-2 binaries to prove the warm-cache-rebuilt compiler is correct, not
just present. Prints WARMCACHE_FIXPOINT_OK on success.
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