fix(listener/tap): support custom TAP adapter name on Windows (issue #772)

The songgao/water library's InterfaceName parameter is a selection filter
that searches the Windows registry for an existing adapter with a matching
friendly name — it cannot name a new adapter during creation. Passing
name=tap0 caused the library to fail before even creating the adapter.

Fix by omitting InterfaceName from water.New(), letting Windows auto-name
the adapter, then renaming via netsh if a custom name was specified.

Also add directory validation to file handler to catch misconfig early.

handler/file/handler.go

@@ -4,6 +4,7 @@ package file
 
 import (
 	"context"
+	"fmt"
 	"io"
 	"net"
 	"net/http"
@@ -57,6 +58,14 @@ func (h *fileHandler) Init(md md.Metadata) (err error) {
 		return
 	}
 
+	if h.md.dir != "" {
+		if info, err := os.Stat(h.md.dir); err != nil {
+			return fmt.Errorf("file handler: directory %q: %w", h.md.dir, err)
+		} else if !info.IsDir() {
+			return fmt.Errorf("file handler: %q is not a directory", h.md.dir)
+		}
+	}
+
 	fs := http.FileServer(http.Dir(h.md.dir))
 	h.handler = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		if r.Method == http.MethodPut {