Report the directories a growing tree could not watch

Extending an automatically watched tree to a new directory swallowed
every error, so a reached watch limit or an unreadable directory left
part of the tree unwatched without any sign. No call of the consumer is
running at that moment, so the failures now arrive in the event stream
as InotifyEvent.watchFailed, after the event that triggered the
extension. The library watches what it can first: a reached limit ends
the attempt, an unreadable directory is skipped with its subtree, and a
directory that vanished in between is not reported.

The buffer now carries the library's own events next to the kernel's,
which keeps them in order and hides the stream's element type. The
test runner drops root's DAC capabilities so that an unreadable
directory can be tested.

Sources/TaskCLI/Test Command.swift

@@ -28,6 +28,9 @@ struct TestCommand: AsyncParsableCommand {
 				"-v", "\(currentDirectory):/code",
 				"-v", "swift-inotify-build-cache:/code/.build",
 				"--security-opt", "systempaths=unconfined",
+				// Root ignores directory permissions unless these are dropped; a
+				// test relies on an unreadable directory.
+				"--cap-drop", "DAC_OVERRIDE", "--cap-drop", "DAC_READ_SEARCH",
 				"--platform", Docker.getLinuxPlatformStringWithHostArchitecture(),
 				"-w", "/code", "swift:latest",
 				"/bin/bash", "-c", "swift test --skip InotifyLimitTests && swift test --skip-build --filter InotifyLimitTests"