A source-agnostic fetch-mock harness (test-support.ts) stubs the global fetch with an ordered route table, so an importer can be driven against captured fixtures with zero live network — an unstubbed fetch throws (blockRealFetch). On top of it, regression tests for the pre-existing raco / ical / webpage importers. This is the reusable test infrastructure factored out of #58 so the guild.host importer PR carries only its own code and tests. Land this first; #58 rebases onto it and keeps just the guild importer + its tests. 19 tests, `npm test` green, no network.