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.

README.md

@@ -46,6 +46,8 @@ for await event in await inotify.events {
         print("Event at \(change.path): \(change.mask)")
     case .queueOverflow:
         print("The kernel dropped events; rescan if you must not miss changes.")
+    case .watchFailed(let path, let error):
+        print("Changes below \(path) go unreported: \(error)")
     }
 }
 ```
@@ -84,6 +86,8 @@ Items that already exist inside a directory that appears this way never produce
 
 When a watched directory is moved out of the tree, the watches on it and on its subdirectories are removed, so no events are reported under the stale path.
 
+Extending the watch to a new directory can fail, typically because the user's watch limit (`fs.inotify.max_user_watches`) is reached or the directory is not readable. The library then watches what it can and delivers `InotifyEvent.watchFailed(path:error:)` for each directory it could not watch, so changes below that path are known to go unreported. A directory that vanished before it could be watched is not reported. The explicit `addRecursiveWatch` and `addWatchWithAutomaticSubtreeWatching` calls, by contrast, either watch the whole tree or throw and leave no watch behind.
+
 ## Excluding Items
 
 You can tell the `Inotify` actor to ignore certain file or directory names, either exactly or by a shell pattern. Excluded items are skipped during recursive directory resolution (so no watch is installed on them), never get a watch when they appear later, and are silently dropped from the event stream: