nx_loader.nx

nx_loader.nx -- ELF segment loader for nx_rv64_sim. Closes the loop: nx_elf_read parses, nx_loader copies PT_LOAD segments into a memory buffer, nx_rv64_sim executes from there. With these three modules we can run an RV64 ELF entirely in NishiLang -- no qemu, no docker, no Linux process boundary. This is THE off-C bootstrap-verification capability. What we do: 1. Parse the ELF header. 2. For each program header: * If type = PT_LOAD, copy `p_filesz` bytes from ELF[p_offset..p_offset+p_filesz) to mem[p_vaddr...] * Then zero `p_memsz - p_filesz` bytes (BSS region). 3. Return the entry point so the simulator can set PC. Memory model: we treat `mem_base[0..mem_size)` as a flat region where addresses are file-vaddrs. If p_vaddr is 0x10000, then mem_base[0x10000] is where the segment lands. The caller allocates a mem buffer big enough to hold all loaded segments; today that means `>= max_vaddr + max_segment_size`. PT_LOAD is the only program-header type we handle; PT_PHDR / PT_INTERP / etc. are silently skipped (irrelevant for static RV64 ELFs from elf_writer).