nx_doc_mainscan.nx

nx_doc_mainscan.nx -- DOCTOR diagnosis for a NEW failure class discovered live this session: nxasm exits rc=102 "UNDEFINED label: main" when a LIBRARY module (functions only, no `func main`) is built as a standalone executable (the linker has no entry point). Seen real: `nx_sov_build_run nx_tls13` -> rc=102, because nx_tls13.nx is the TLS-1.3 record-layer LIBRARY (consumed by nx_pub_tls_serve / the gates, which DO build green). This is a DIFFERENT class from compiler-level CC-UNDEFFN ("call to undefined function: <name>"): it is an ASSEMBLER-level missing entry-point. READ-ONLY BY CONSTRUCTION: it does NOT fabricate a `main` (that would be a fake program and could mask a genuinely missing entry point) -- it DIAGNOSES "this source has no main; if it is a library, build it through its consumer/gate, not standalone." Reuses kw_eq + dhl_isident/dhl_name_at from nx_doc_kwscan. ORIGINAL