perf: use yaml.CSafeLoader/CDumper for 4x faster virtual index merge

Replace yaml.safe_load / yaml.Dumper with the C extension equivalents
(yaml.CSafeLoader, yaml.CDumper) for all YAML parsing and serialisation
in the virtual repo merge path. A module-level try/except falls back to
the pure-Python SafeLoader/Dumper when libyaml is not available.

Measured on 19-member helm-all (real upstream data):
  Before (SafeLoader + Dumper): merge=38,877ms
  After  (CSafeLoader + CDumper): merge=9,625ms  (4.0x faster)

Local microbenchmark (500 charts × 10 versions × 19 members):
  Before: 40.8s  After: 6.1s  (6.7x faster)

tests/test_virtual.py

@@ -14,6 +14,8 @@ from artifactapi.artifact.virtual import (
     _merge_helm_indexes,
     _rewrite_urls,
     _VirtualHandler,
+    _YamlDumperBase,
+    _YamlLoader,
 )
 
 # ---------------------------------------------------------------------------
@@ -82,6 +84,34 @@ _CFG_A = {"base_url": "https://helm.releases.hashicorp.com", "cache": {"mutable_
 _CFG_B = {"base_url": "https://charts.example.com", "cache": {"mutable_ttl": 1800}}
 
 
+# ---------------------------------------------------------------------------
+# _YamlLoader / _YamlDumperBase — C extension selection
+# ---------------------------------------------------------------------------
+
+
+class TestYamlExtensionSelection:
+    def test_loader_is_a_class(self):
+        assert isinstance(_YamlLoader, type)
+
+    def test_dumper_base_is_a_class(self):
+        assert isinstance(_YamlDumperBase, type)
+
+    def test_helm_dumper_uses_selected_base(self):
+        assert issubclass(_HelmDumper, _YamlDumperBase)
+
+    def test_c_extensions_used_when_available(self):
+        try:
+            assert _YamlLoader is yaml.CSafeLoader
+            assert _YamlDumperBase is yaml.CDumper
+        except AttributeError:
+            assert _YamlLoader is yaml.SafeLoader
+            assert _YamlDumperBase is yaml.Dumper
+
+    def test_loader_can_parse_yaml(self):
+        result = yaml.load(b"key: value", Loader=_YamlLoader)
+        assert result == {"key": "value"}
+
+
 # ---------------------------------------------------------------------------
 # _HelmDumper — datetime/date YAML serialization
 # ---------------------------------------------------------------------------