wwtest: native package-test coordinator

Single-directory package-test planner: discovers *.ww, groups white-box
and external <pkg>_test sources, composes the combined test package,
builds it through the sibling ww driver, and runs it with the runtime's
-package/-list/-timeout-ms contract. test/package holds its E2E corpus
and the manual runtime fixture set.

test/package/cases/nonzero/nonzero_test.ww

@@ -0,0 +1,5 @@
+package nonzero_test;
+
+import os;
+
+@test fn deliberate_nonzero_exit() void = { os.exit(7); };