Deliver an event enum with the queue overflow as its own case

The stream's element is now the enum InotifyEvent, and the struct that
describes a change to a watched item is FileSystemEvent. A queue
overflow was an event with descriptor -1 and an empty path that every
consumer had to know about; as a case, the compiler makes them handle
it. The enum is also where failed watches of a growing tree will be
reported, since no call site can catch them.

Tests/InotifyIntegrationTests/InotifyLimitTests.swift

@@ -50,7 +50,7 @@ struct InotifyLimitTests {
 					var received = 0
 					for await event in await watcher.events {
 						received += 1
-						if event.mask.contains(.queueOverflow) { return (event, received) }
+						if case .queueOverflow = event { return (event, received) }
 					}
 					return (nil, received)
 				}
@@ -65,9 +65,7 @@ struct InotifyLimitTests {
 				overflowTask.cancel()
 				let (overflow, received) = await overflowTask.value
 
-				#expect(overflow != nil, "Expected a queue overflow event after \(index) file creations and \(received) received events")
-				#expect(overflow?.watchDescriptor == -1)
-				#expect(overflow?.path == "")
+				#expect(overflow == .queueOverflow, "Expected a queue overflow event after \(index) file creations and \(received) received events")
 			}
 		}
 	}

Tests/InotifyIntegrationTests/Utilities/getEventsForTrigger.swift

@@ -6,6 +6,7 @@ enum RecursivKind {
 	case withAutomaticSubtreeWatching
 }
 
+/// The file system events an instance delivers around `trigger`.
 func getEventsForTrigger(
 	in dir: String,
 	mask: InotifyEventMask,
@@ -13,6 +14,27 @@ func getEventsForTrigger(
 	exclude: [String] = [],
 	excludePatterns: [String] = [],
 	trigger: @escaping (String) async throws -> Void,
+) async throws -> [FileSystemEvent] {
+	let events = try await getInotifyEventsForTrigger(
+		in: dir,
+		mask: mask,
+		recursive: recursive,
+		exclude: exclude,
+		excludePatterns: excludePatterns,
+		trigger: trigger
+	)
+	return events.compactMap(\.fileSystemEvent)
+}
+
+/// Everything an instance delivers around `trigger`, including the
+/// events that are not about a file system item.
+func getInotifyEventsForTrigger(
+	in dir: String,
+	mask: InotifyEventMask,
+	recursive: RecursivKind = .nonrecursive,
+	exclude: [String] = [],
+	excludePatterns: [String] = [],
+	trigger: @escaping (String) async throws -> Void,
 ) async throws -> [InotifyEvent] {
 	let watcher = try Inotify()
 	await watcher.exclude(names: exclude)
@@ -41,3 +63,10 @@ func getEventsForTrigger(
 	eventTask.cancel()
 	return await eventTask.value
 }
+
+extension InotifyEvent {
+	var fileSystemEvent: FileSystemEvent? {
+		if case .fileSystem(let event) = self { return event }
+		return nil
+	}
+}