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/dep/contest.ww

@@ -0,0 +1,4 @@
+package dep;
+
+// A production filename ending in "test.ww" is not a test by itself.
+export fn bonus() int = { return 2; };

test/package/dependency/dep/dep.ww

@@ -0,0 +1,3 @@
+package dep;
+
+export fn value() int = { return 40; };

test/package/dependency/dep/deptest.ww

@@ -0,0 +1,7 @@
+package dep;
+
+// Noncanonical migration spelling: the actual line-leading @test makes this
+// test-only.  It must be excluded when dep is imported by the root package.
+@test fn imported_dependency_test_must_not_leak() void = {
+	assert(false);
+};

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);
+};