feat(06-02): implement JSONFormatter with Unmask support

- Renders findings as 2-space indented JSON array
- Honors Options.Unmask for key field exposure
- Omits empty verify fields via json omitempty
- Registers under "json" in output registry

pkg/output/json_test.go

@@ -142,8 +142,12 @@ func TestJSONFormatter_Indented(t *testing.T) {
 	if err := (JSONFormatter{}).Format([]engine.Finding{f}, &buf, Options{}); err != nil {
 		t.Fatalf("Format returned error: %v", err)
 	}
-	// Expect 2-space indent on at least one property line.
-	if !strings.Contains(buf.String(), "\n  \"provider\"") {
-		t.Errorf("output not indented with 2 spaces:\n%s", buf.String())
+	// Expect 2-space indent: array items at 2 spaces, nested fields at 4 spaces.
+	s := buf.String()
+	if !strings.Contains(s, "\n  {") {
+		t.Errorf("array item not indented with 2 spaces:\n%s", s)
+	}
+	if !strings.Contains(s, "\n    \"provider\"") {
+		t.Errorf("nested field not indented with 4 spaces:\n%s", s)
 	}
 }