Mark @ResourceWrapper-generated extensions and Definition types nonisolated

[NicoMulet]

Fixes #19

TL;DR

Mark @ResourceWrapper's generated extensions and Definition/BodyDefinition types nonisolated so the macro output no longer depends on the consuming module's default actor-isolation setting.

Context

Xcode 26's "Approachable Concurrency" makes @MainActor the default isolation for any declaration that doesn't opt out explicitly. @ResourceWrapper generates several extension Person: ... conformances and a nested Definition struct without marking them nonisolated, so under that default they inherit @MainActor isolation. Since ResourceDefinition/ResourceDefinitionProviding are ordinary nonisolated protocols, this produces:

error: conformance of 'Person' to protocol 'ResourceDefinitionProviding' crosses into main actor-isolated code and can cause data races

Reported in #19, where the reporter found that adding nonisolated to their own model works around it. Resource models are plain data-transfer types encoded/decoded across concurrency domains, so they shouldn't be actor-isolated regardless of what isolation default the consuming module picked — the fix belongs in the macro.

Summary of Changes

• @ResourceWrapper now emits nonisolated on all 5 generated extensions (ResourceDefinitionProviding, ResourceIdentifiable, ResourceLinkageProviding, Codable, and the body-factory extension).
• The generated Definition and BodyDefinition struct declarations are also marked nonisolated, since nested type declarations are independent isolation-inference roots and don't inherit isolation from their enclosing (now-nonisolated) extension.
• Attributes/Relationships are intentionally left unmarked: they hold mutable var stored properties, and nonisolated on a mutable stored property is a hard error in the Swift 6 language mode. They don't need it anyway — the compiler error was specifically about Definition's conformance, not these nested value types.
• Updated the ResourceWrapperMacroTests expansion snapshots to match.

How to Test

Verified against the reporter's exact repro (Person with id/firstName/lastName/twitter) using swift build -Xswiftc -swift-version -Xswiftc 6 -Xswiftc -default-isolation -Xswiftc MainActor on a client target depending on this branch:

• Before this change: fails with the reported "crosses into main actor-isolated code" error.
• After this change: builds cleanly.

Also ran the full existing test suite (swift test), all green with no new warnings.

Demo

@ResourceWrapper(type: "people")
struct Person: Equatable {
    var id: String

    @ResourceAttribute var firstName: String
    @ResourceAttribute var lastName: String
    @ResourceAttribute var twitter: String?
}
// Compiles under `-default-isolation MainActor` (Xcode 26's Approachable
// Concurrency default) without needing to mark `Person` itself `nonisolated`.