fix(test): expect_eq on strings compares their text and prints both on a failure

It lowered to an i32 compare of two pointers, which the IR refused. Two strings with the same text
are equal now, a null only to a null, and a failure says expect_eq failed (got "camp", want
"lake"). examples/library/testing_strings.ludic (two tests fail on purpose, and the output is
checked); ludic.base's actions_test uses it again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.base/tests/actions_test.ludic

@@ -25,6 +25,6 @@ program ActionsTest {
     core_add(b, s)
     core_tick_all(b, tick_new(0.016, 1, 0.0))
     expect(d.open)
-    expect(d.seen == "o")
+    expect_eq(d.seen, "o")
   }
 }