nx_gzip_lib_probe.nx

nx_gzip_lib_probe.nx -- proves nx_gzip_lib.nx is IMPORTABLE and that gz_compress links and works. WHY THIS EXISTS: /api/build compiles its target as a PROGRAM, so a no-main lib cannot be built standalone -- "no artifact" from `build target=nx_gzip_lib` is therefore NOT evidence the lib is broken. The only honest test of an importable lib is to build a consumer of it. This is that consumer, and it is also the CONTROL that separates "my lib is bad" from "the build lane is stuck". ANTI-VACUITY: a probe that only checked rc=0 would pass if gz_compress returned garbage, so it asserts the RFC-1952 magic bytes AND that the output is genuinely smaller than the input. A stub returning 0, or copying the input through, cannot pass this.