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
unkinben
15f934cd0b