Add conditional Sendable conformances

[NicoMulet]

TL;DR

Add conditional Sendable conformances.

Context

With Swift 6's strict concurrency checking (and Xcode 26.5's default warnings), a value produced on the main actor and handed to a non-isolated @Sendable closure must conform to Sendable. None of the library's resource types declared Sendable, which forced every downstream app to add per-target retroactive conformances such as:

 extension JSONAPI.ResourceBody: @retroactive @unchecked Sendable {}

Conditional Sendable belongs in the library, mirroring how it already conditionally provides Equatable and Codable.

Summary of Changes

• Add unconditional Sendable conformance to Unit, ResourceIdentifier, RelationshipOne, RelationshipMany — their stored properties are always Sendable.
• Add conditional Sendable conformance to Resource, ResourceBody, InlineRelationshipOne, InlineRelationshipMany, InlineRelationshipOptional, CompoundDocument, DefaultEmpty — gated on
  the generic parameters that show up in stored properties.
• @ResourceWrapper macro now propagates Sendable to the generated Attributes and Relationships structs when the wrapper itself conforms to Sendable (same detection pattern already used
  for Equatable).
• Add a public init() to Unit so it's constructible from outside the module (used by the new tests).
• Add SendableTests (11 type-witness checks) and a testResourceWrapperPropagatesSendable macro snapshot test to lock the behavior in.

How to Test

N/A — covered by the new tests.

Demo

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

      @ResourceAttribute var firstName: String
      @ResourceRelationship var related: Person?
  }

  // `Person.Body` is now `Sendable`, so values can flow into a non-isolated
  // `@Sendable` closure without "sending main actor-isolated 'body' risks
  // causing data races" warnings.
  let body = Person.createBody(id: "1", firstName: "Dan")
  Task.detached {
      await send(body)
  }

[alanf]

LGTM with a question about phantom copy

[alanf]

// Destination is phantom

I have no idea what this means...what does phantom mean here?

[NicoMulet]

Destination is not really stored inside RelationshipMany, it is only here to provide the ID type from the ResourceLinkageProviding. I think it fits the definition of Swift Phantom Types, but happy to change that if you think that's not relevant

[alanf]

TIL!