[kylebrowning]

Cleanups from the codebase review. No behavior change; the existing header-propagation and encoding tests cover each path.

Changes

• APNSRequest is a plain struct. The private copy-on-write class with @unchecked Sendable, a copy() method, and eight hand-written isKnownUniquelyReferenced accessor pairs provided exactly the value semantics a struct with stored public var properties already has. Public API is unchanged. Net 160 lines removed.
• APNSClient.send uses APNSRequest.headers. The NIO client had its own copy of the apns-* header mapping while the URLSession client used the shared one. A new header added to one could silently miss the other. Both now derive from the same property, which sendBroadcast already did.
• APNSAlertNotification EmptyPayload init delegates to the designated init instead of duplicating its body, so a new aps field only needs to be threaded through once. The default aps storage that every init immediately overwrote is gone.
• APNSBroadcastClient Sendable conformance is unconditional. The class already constrained Decoder and Encoder to Sendable, so the conditional extension was redundant.

Full suite: 154 tests, 0 failures.

Not included

The alert notification traps with a precondition on an out-of-range relevance score while Live Activity notifications accept any value. Changing either direction is a behavior decision, so I left it alone.