nx_env.nx

nx_env.nx -- environment variable lookup over the System V envp. At program entry the kernel hands us argc + argv + envp. envp is a NULL-terminated array of "KEY=VALUE" strings. This module searches envp for a given key and returns the value pointer (NUL- terminated) or NULL. Callers (toolchain): NX_NXC_PATH, NX_DEBUG, HOME, PATH, etc. We don't mutate envp -- this is a pure read interface. Setenv would require redirecting stdlib calls, which the off-C runtime doesn't have anyway.