buildSpigot
private fun buildSpigot(mcUri: String, version: String, cacheDir: Path, mappings: Path, symbolsDir: Path)
Spigot variant: Mojang's client_mappings (named<->obf) + BuildData's class csrg (obf->spigot), pivoted on obf. Two sources for the same reason fabric needs two — BOTH axes move on a spigot runtime. Members are the obf names, so proguard carries them; classes are spigot's OWN names (BlockPos is BlockPosition), which no Mojang artifact can express, so they take the second source.