Function validate_contribution 

pub fn validate_contribution(
    store: &Store,
    plugin_id: &str,
    batch: &Value,
) -> Result<Value>

Validate a plugin contribution batch (§24): entity ids and kinds present, relationship endpoints resolve within (batch entities + graph), custom ontology namespaced plugin:<id>/..., evidence ids present.

Returns the normalized batch on success; broken plugins fail BEFORE any write — the model is never left half-committed.