buildArtifacts
fun buildArtifacts(clientTxt: String, secondSource: String, runtimeMcUri: String, outMappings: String, outSymbolsDir: String)
Assemble outMappings (tiny v2, named<->intermediary) from the downloaded clientTxt (Mojang proguard)
secondSource, then reverse-remap runtimeMcUri (the loader's view of the runtime jar) into
<outSymbolsDir>/mc-symbols.jar(mojmap names, for the Kotlin compiler). Callers gate on the outputs already existing.
secondSource carries the runtime's CLASS names, which proguard cannot: a fabric intermediary jar, or a spigot .csrg, told apart by extension. Both runtimes move both axes, so both need one.