Function validate_code_rules 

pub fn validate_code_rules(
    action: ActionKind,
    target_class: &str,
    evalset_hash: Option<&str>,
) -> Result<()>

Rule E1’s structural checks (§7.4), applied when a draft is stamped and re-checked when a stored grain is loaded (a hand-authored grain must not bypass the rule):

• code_revision ⇔ a tool: target, and it MUST pin an evalset hash.
• An evalset: target is its own always-human-approved class: never a code_revision, and never itself pinned (the gate cannot gate itself).
• No other target class carries a pin.