fix(apple): make receive-cancel actually cancel the transfer

The Cancel button on an in-progress receive did nothing. CoreRepository
funnelled every core call through one serial DispatchQueue, but `receive`
is a blocking core call that occupies that queue for the whole transfer.
The tapped `cancelTransfer` was enqueued behind the in-flight `receive` on
the same serial queue, so it could never run until `receive` returned —
which it never would, because it was waiting to be cancelled. A deadlock
the button couldn't escape.

The Rust core is explicitly designed for cancel to arrive from another
thread mid-receive (VnidropCore.block_on uses a shared runtime handle for
exactly this). Extracts the two-lane dispatch into a CoreDispatcher: a
serial lane for ordered calls and a separate concurrent lane for
interrupt-style calls, and routes cancel through the latter so the signal
reaches the core and unblocks the receive.

Adds CoreDispatcherTests, including a regression guard that an interrupt
completes while the serial lane is blocked.

apple/VniDrop/Core/CoreDispatcher.swift

@@ -0,0 +1,41 @@
+import Foundation
+
+/// Dispatch-queue labels for the core's serial and interrupt lanes.
+enum QueueLabel {
+	static let core = "com.vnidrop.core"
+	static let interrupt = "com.vnidrop.core.interrupt"
+}
+
+/// Two-lane dispatcher for blocking core calls.
+///
+/// `run` serializes calls on one queue so the core is driven from a single lane.
+/// `runInterrupt` uses a *separate* concurrent lane, so an interrupt-style call
+/// (cancel) can reach the core while a blocking call (`receive`) still occupies
+/// the serial lane. The core is internally synchronized and explicitly supports
+/// cancel arriving from another thread mid-receive (see VnidropCore.block_on);
+/// a single shared queue would deadlock it.
+final class CoreDispatcher: Sendable {
+	private let serialQueue: DispatchQueue
+	private let interruptQueue: DispatchQueue
+
+	init(label: String = QueueLabel.core, interruptLabel: String = QueueLabel.interrupt) {
+		serialQueue = DispatchQueue(label: label, qos: .userInitiated)
+		interruptQueue = DispatchQueue(label: interruptLabel, qos: .userInitiated, attributes: .concurrent)
+	}
+
+	/// Runs a blocking core call on the serial lane and hops the result back.
+	func run<T: Sendable>(_ block: @escaping @Sendable () throws -> T) async -> Result<T, Error> {
+		await withCheckedContinuation { continuation in
+			serialQueue.async { continuation.resume(returning: Result { try block() }) }
+		}
+	}
+
+	/// Like `run`, but off the serial lane so it can interrupt a blocking call in
+	/// flight there (e.g. cancel a `receive`). Only use for core calls that are
+	/// safe to run concurrently with another core call.
+	func runInterrupt<T: Sendable>(_ block: @escaping @Sendable () throws -> T) async -> Result<T, Error> {
+		await withCheckedContinuation { continuation in
+			interruptQueue.async { continuation.resume(returning: Result { try block() }) }
+		}
+	}
+}