1.0 quality polish — code review, tests, bug fixes, documentation (#32)

Comprehensive quality pass targeting 1.0 release:

- Code review and refactoring
- Fix open bugs (#14, #16, #23)
- Expand test coverage
- Lint clean
- README update with build instructions (#9)
- Documentation improvements

Branched from `next` (active dev branch).

Reviewed-on: #32
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

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())
+}