Birth test/sep/ (single-file ww observers on test/testenv, run by the
new SEP_WW_TARGETS pattern rule under test-compiler) with
sepbuild_test.ww: the build_one_sep real-chain gate incl. the
bodies==.wwi keystone recompile, the #69 root-without--I contract, the
#70 aggregate-def .wwi prototypes, and the #95 spread-marker
round-trip. Every carrier assertion has a successor row; the C
cleanup-accounting ledgers collapse into testenv.clean.
