perf: use yaml.CSafeLoader/CDumper for 4x faster virtual index merge (#39)

Closes #34

## Summary

- At module load time, a `try/except` selects `yaml.CSafeLoader` / `yaml.CDumper` (C extensions) when libyaml is available, otherwise falls back to `yaml.SafeLoader` / `yaml.Dumper`
- `_HelmDumper` inherits from whichever dumper base was selected — custom datetime/date representers are registered the same way as before
- `_merge_helm_indexes` uses `yaml.load(raw_data, Loader=_YamlLoader)` instead of `yaml.safe_load`
- No change to `yaml.dump(...)` call — it already passes `Dumper=_HelmDumper`, which now inherits from the C base when available
- Five new tests in `TestYamlExtensionSelection` cover: loader/dumper base are classes, `_HelmDumper` inherits from the selected base, C extensions used when available, loader can parse YAML

## Measured performance gain

19-member `helm-all` virtual repo, real upstream data, Docker (AlmaLinux 9):

| | `merge=` time |
|---|---|
| Before (SafeLoader + Dumper) | **38,877ms** |
| After (CSafeLoader + CDumper) | **9,625ms** |
| Speedup | **4.0×** |

Local microbenchmark (500 charts × 10 versions × 19 members, 3 runs avg):
- Before: **40.8s** → After: **6.1s** (**6.7×** faster)

## Test plan

- [x] 283 unit tests pass (`make test`)
- [x] Wheel builds cleanly (`uv build --wheel`)
- [x] C extension confirmed available in AlmaLinux 9 container: `yaml.CSafeLoader: <class 'yaml.cyaml.CSafeLoader'>`
- [x] Baseline Docker timing measured with pure-Python path forced: merge=38,877ms
- [x] After Docker timing measured with C extension path: merge=9,625ms

Reviewed-on: #39

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
 # ---------------------------------------------------------------------------