ir_validate.nx

ir_validate.nx -- alias stub. Migration to nx_-prefixed canonical name. Consumers via "import \"ir_validate.nx\"" resolve to nx_ir_validate.nx through NishiLang's textual import splicing with path dedup (runtime/import.nx I1). Eliminates duplicate-symbol errors when both names appear in the transitive import tree (e.g., nxc.nx imports ir_validate.nx directly AND nx_regalloc.nx imports nx_ir_validate.nx). Deprecation: delete this stub when every "ir_validate.nx" consumer has migrated to "nx_ir_validate.nx".