Function parse 

pub fn parse(yaml: &str) -> Result<PackManifest, PackParseError>

Parse a pack.yaml buffer into a PackManifest.

The entry point:

1. Pre-scans for YAML anchor / alias events and rejects them.
2. Deserializes into a permissive raw map.
3. Validates schema_version and name.
4. Segregates known fields from x-* extensions; rejects any other unknown top-level key.
5. Key-dispatches actions and teardown via Action::from_yaml.

Errors

Any structural violation surfaces as a PackParseError variant with enough context for a CLI consumer to point at the offending key.