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/dependency/root/root.ww

@@ -0,0 +1,5 @@
+package root;
+
+import dep;
+
+fn answer() int = { return dep.value() + dep.bonus(); };

test/package/dependency/root/root_test.ww

@@ -0,0 +1,5 @@
+package root;
+
+@test fn imported_production_only() void = {
+	assert(answer() == 42);
+};