Function farm_key 

pub fn farm_key(tree_json: &str, phrase_json: &str) -> String

The content address of (tree_json, phrase_json) without rendering it — what a caller asks the persistent cache about before paying for a render.

Returns "" if either argument fails to parse, which the caller should treat as a miss rather than an error: the render path validates its own inputs and is the one place allowed to reject them.