nx_elf_read.nx

nx_elf_read.nx -- read & parse ELF64 binaries we produce. Closes the loop with elf_writer + nx_strtab + nx_symtab + nx_shtab + nx_dwarf_line: this module READS what those modules WRITE. Foundation for sovereign objdump, nm, readelf, and (future) nx_dbg.nx debugger. Without nx_elf_read we depend on third-party tools to inspect our output -- a sovereignty hole. With it, every byte produced is also auditable in pure NishiLang. What we parse: * ELF header (64 bytes): magic, class, machine, entry, phoff, shoff, ehsize, phentsize, phnum, shentsize, shnum, shstrndx. * Program headers (each 56 bytes): PT_LOAD segments etc. * Section headers (each 64 bytes): name, type, flags, addr, offset, size, link, info, align, entsize. * Symbol tables (each Elf64_Sym = 24 bytes). * String tables (lookup by offset). NOT YET (separate commits): * .debug_line VM execution (decode line-program back to (addr -> file/line) tuples). Land with nx_dbg.nx. * .debug_info DIE walker. * Relocations (.rela parsing) -- when nx_reloc.nx ships.