refactor(apple): type core event phase/kind/direction as enums

Replaces the stringly-typed transfer-event phase/kind/direction values
throughout the progress-derivation logic with EventPhase, EventKind and
EventDirection enums (String-backed to match the core's wire values).

CoreEventModel keeps the raw wire strings as a faithful boundary DTO but
exposes typed eventPhase/eventKind/eventDirection accessors; all logic —
progressForTransfer/Receiver, humanProgressLabel, aggregateReceiverProgress,
the refresh trigger, and the SendScreen snapshots — now compares enum cases
instead of literals. TransferProgress.phase/kind are the enums directly, so
constructions read `phase: .transfer, kind: .progress`. The two ad-hoc
phase/kind Sets collapse into "is a recognized case" (non-nil) checks.

apple/Tests/ProgressDerivationTests.swift

@@ -57,7 +57,7 @@ final class ProgressDerivationTests: XCTestCase {
 			remoteEndpointId: "peer-a",
 			totalSizeHint: 100
 		)
-		XCTAssertEqual(progress?.kind, "completed")
+		XCTAssertEqual(progress?.kind, .completed)
 		XCTAssertEqual(progress?.labelKey, L10n.Progress.completed)
 		XCTAssertEqual(progress?.progress, 1)
 	}