assembleMappings

Assemble outMappings from the downloaded clientTxt (Mojang proguard) + secondSource, which carries the runtime names proguard cannot — told apart by extension: MCPConfig's joined.tsrg (forge's srg members, out as TSRG2), or spigot BuildData's .csrg / the fabric intermediary jar (both runtimes move both axes; out as tiny v2). The MC-version half of the bundle: nothing here depends on the loader.