Add CanWatch impls to physical and composite FS

src/Zio/FileSystems/AggregateFileSystem.cs

@@ -406,6 +406,13 @@ protected override UPath ConvertPathFromDelegate(UPath path)
         // Watch API
         // ----------------------------------------------
 
+        /// <inheritdoc />
+        protected override bool CanWatchImpl(UPath path)
+        {
+            // Always allow watching because a future filesystem can be added that matches this path.
+            return true;
+        }
+
         /// <inheritdoc />
         protected override IFileSystemWatcher WatchImpl(UPath path)
         {

src/Zio/FileSystems/ComposeFileSystem.cs

@@ -197,6 +197,12 @@ protected override IEnumerable<UPath> EnumeratePathsImpl(UPath path, string sear
         // ----------------------------------------------
         // Watch API
         // ----------------------------------------------
+
+        /// <inheritdoc />
+        protected override bool CanWatchImpl(UPath path)
+        {
+            return NextFileSystemSafe.CanWatch(ConvertPathToDelegate(path));
+        }
 
         /// <inheritdoc />
         protected override IFileSystemWatcher WatchImpl(UPath path)

src/Zio/FileSystems/MountFileSystem.cs

@@ -627,6 +627,13 @@ IEnumerable<UPath> EnumeratePathFromFileSystem(UPath subPath, bool failOnInvalid
             }
         }
 
+        /// <inheritdoc />
+        protected override bool CanWatchImpl(UPath path)
+        {
+            // Always allow watching because a future filesystem can be added that matches this path.
+            return true;
+        }
+
         /// <inheritdoc />
         protected override IFileSystemWatcher WatchImpl(UPath path)
         {

src/Zio/FileSystems/PhysicalFileSystem.cs

@@ -510,6 +510,17 @@ protected override IEnumerable<UPath> EnumeratePathsImpl(UPath path, string sear
         // ----------------------------------------------
         // Watch API
         // ----------------------------------------------
+
+        /// <inheritdoc />
+        protected override bool CanWatchImpl(UPath path)
+        {
+            if (IsWithinSpecialDirectory(path))
+            {
+                return SpecialDirectoryExists(path);
+            }
+
+            return Directory.Exists(ConvertPathToInternal(path));
+        }
 
         /// <inheritdoc />
         protected override IFileSystemWatcher WatchImpl(UPath path)