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/bad_external/bad_external.ww

@@ -0,0 +1,3 @@
+package bad_external;
+
+export fn value() int = { return 1; };

test/package/bad_external/wrong_test.ww

@@ -0,0 +1,3 @@
+package unrelated_test;
+
+@test fn wrong_external_name() void = { assert(true); };