`SWIFT_DEFAULT_ACTOR_ISOLATION = MainActor` makes **every unannotated type implicitly `@MainActor`** — including pure data→file exporters, geometry builders, value-type models, and plain DTOs written to run off-main. The naive fixes are both wrong: `await MainActor.run` hops heavy compute onto the main thread (UI jank); `@preconcurrency` / `nonisolated(unsafe)` hides real data races.
