Fix BaseURL.JoinPath encoding slashes in paths, add URL tests

JoinPath used url.PathEscape on the entire path which encoded slashes
as %2F. Now encodes each segment individually. Add tests for all URL
types.

mfer/url.go

@@ -27,8 +27,12 @@ func (b BaseURL) JoinPath(path RelFilePath) (FileURL, error) {
 		base.Path += "/"
 	}
 
-	// Parse and encode the relative path
-	ref, err := url.Parse(url.PathEscape(string(path)))
+	// Encode each path segment individually to preserve slashes
+	segments := strings.Split(string(path), "/")
+	for i, seg := range segments {
+		segments[i] = url.PathEscape(seg)
+	}
+	ref, err := url.Parse(strings.Join(segments, "/"))
 	if err != nil {
 		return "", err
 	}

mfer/url_test.go

@@ -0,0 +1,44 @@
+package mfer
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestBaseURLJoinPath(t *testing.T) {
+	tests := []struct {
+		base     BaseURL
+		path     RelFilePath
+		expected string
+	}{
+		{"https://example.com/dir/", "file.txt", "https://example.com/dir/file.txt"},
+		{"https://example.com/dir", "file.txt", "https://example.com/dir/file.txt"},
+		{"https://example.com/", "sub/file.txt", "https://example.com/sub/file.txt"},
+		{"https://example.com/dir/", "file with spaces.txt", "https://example.com/dir/file%20with%20spaces.txt"},
+	}
+
+	for _, tt := range tests {
+		t.Run(string(tt.base)+"+"+string(tt.path), func(t *testing.T) {
+			result, err := tt.base.JoinPath(tt.path)
+			require.NoError(t, err)
+			assert.Equal(t, tt.expected, string(result))
+		})
+	}
+}
+
+func TestBaseURLString(t *testing.T) {
+	b := BaseURL("https://example.com/")
+	assert.Equal(t, "https://example.com/", b.String())
+}
+
+func TestFileURLString(t *testing.T) {
+	f := FileURL("https://example.com/file.txt")
+	assert.Equal(t, "https://example.com/file.txt", f.String())
+}
+
+func TestManifestURLString(t *testing.T) {
+	m := ManifestURL("https://example.com/index.mf")
+	assert.Equal(t, "https://example.com/index.mf", m.String())
+}