Function check_budget 

pub fn check_budget(
    result: &ContextQueryResult,
    query: &ContextQuery,
) -> (bool, String)

Validate a query result against the budget contract (SPEC.md §B1, §B3, §B4). Returns (passed, evidence).

Three distinct promises, deliberately checked separately so a failure says which one broke:

• §B1 the declared costs sum within max_tokens;
• §B3 each declared cost equals the canonical count for its content — this is what turned the check from arithmetic into truth;
• §B4 the frame count respects max_frames.