Add [package].name scope contract to mlf.toml

Introduce a required [package] section whose name field declares the package's NSID prefix. Every .mlf file's namespace must be equal to or descended from it; a mis-scoped file is a miette-rendered error in mlf check rather than a silent pass. mlf init prompts for the name interactively (and derives a placeholder from the directory under --yes); mlf fetch's implicit mlf.toml auto-create is dropped since a package name would need its own prompt — users mlf init first.