Re-source the @error union TypeResolver ladder onto ErrorRouter.Mapping[]

"Does handler H fire on throwable T" has two spellings in the tree. The ErrorRouter.Mapping.match family is the load-bearing one: all three dispatch arms (ErrorRouter.dispatch, ErrorRouter.dispatchToLocalContext, and ChannelCatchArmEmitter, which inlines the loop) go through it, and the @error type’s message: fetcher now walks it too. The second is a hand-rolled instanceof ladder in GraphitronSchemaClassGenerator.buildErrorPolymorphicResolver, which re-implements the same three discriminators in emitter code to pick a union member by source class.

The two have already drifted, which is the concrete reason to collapse them rather than a tidiness argument: ErrorMappingsClassGenerator.bestGuessOrObject falls back to Object.class on a malformed exception class name, while buildErrorPolymorphicResolver calls ClassName.bestGuess bare and throws on the same input. So one SDL shape is a tolerated degradation on the dispatch side and a generator crash on the resolver side.

Collapsing the ladder to a Mapping[] walk leaves one spelling of the predicate and closes that drift. The blast radius is the TypeResolver emission, which is why it was held out of the item that surfaced it (the description: read-side override, shipped; see roadmap/changelog.md) rather than folded in. Note that the ladder is the weaker of the two spellings: it is not a pattern to copy, and any work here moves toward Mapping[], not away from it.