#152 link-the-let-before-its-init miscompile (miscompiled IDENTICALLY on both stages, so byte-id 990-997 was blind; only a runtime value check catches it). Lifted the run fixture into test/lang/letshadow_test.ww (primitive @test value pin; byteid-eligible — symmetric, cs==ww). byteid floor 56->57. Closes the fold-3 #7 umbrella (last C test twin retired).
