Assemble an upload's parts into one file (path in body)

Assemble the parts write_file_part sent into one file: the node concatenates part-0 to part-(part_count-1) in order, checks the total against max_chunked_write_bytes, hashes the result against the sha256 you declare (pass it when you can; a mismatch refuses and keeps the parts), then renames it into place atomically and removes the part directory. Returns {written, path, bytes_written, sha256, sha256_verified, parts, upload_id}; parents are created and an existing file is replaced, like write_file. A refusal says exactly what is wrong: the parts that are missing or at or past part_count, with every part's size on the node, so you re-send only those (same upload_id and part_index) and commit again - nothing is assembled until the set is complete. Committing again after success answers with the same receipt when the file in place hashes to the declared sha256, so a lost answer costs one more call, never a re-upload. To abandon an upload: run_command "rm -rf ~/.givemeanode/uploads/<upload_id>"; part directories untouched for 7 days are removed by the next commit on that node. Calling this on a stopped node wakes it, exactly like write_file, and NOTHING IS ASSEMBLED on the queued/waking answers (written:false); the parts on the disk are intact, so commit again once the node is running. Opens a billed session on the node's mission like any other exec. executable:true sets mode 0755.