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Tester #5506 §2 + Markus #5504 §2: the 409 orchestration was inline in +page.svelte and untested. Extract into a pure module that takes the fetch function as a dependency, so the full happy path / 409 path / 500 path / refetch-fails path / UUID-guard path can be unit-tested with mock Responses. The route file now reads as 12 lines: call the helper, on conflict apply the merged snapshot to local state, re-throw. BlockConflictResolvedError now carries the merged block on its `merged` property so callers don't have to redo the refetch. 6 new unit tests cover every branch. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>