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

src/artifactapi/artifact/virtual.py

@@ -11,8 +11,15 @@ from fastapi import HTTPException, Request, Response
 
 logger = logging.getLogger(__name__)
 
+try:
+    _YamlLoader = yaml.CSafeLoader
+    _YamlDumperBase = yaml.CDumper
+except AttributeError:
+    _YamlLoader = yaml.SafeLoader
+    _YamlDumperBase = yaml.Dumper
 
-class _HelmDumper(yaml.Dumper):
+
+class _HelmDumper(_YamlDumperBase):
     """YAML dumper that serializes datetime/date objects back to ISO 8601 strings.
 
     yaml.safe_load converts timestamp-shaped YAML scalars (e.g. chart `created`
@@ -109,7 +116,7 @@ def _merge_helm_indexes(raw_indexes: list[bytes], member_names: list[str], membe
         base_url = member_cfg.get("base_url", "").rstrip("/")
 
         try:
-            index = yaml.safe_load(raw_data)
+            index = yaml.load(raw_data, Loader=_YamlLoader)
         except Exception as e:
             logger.warning(f"Virtual: failed to parse index.yaml from member '{member_name}': {e}")
             continue