security: audit whether any #[contracterror] Error discriminant values collide within a contract

[Idaonoli]

Scope

This session found at least one contract (denylist-gate, before this pass's fix) had accumulated conflicting Error discriminant numbering from separately-merged features assigning the same number to different variants. Worth a one-time audit across all nine contracts' Error enums to confirm none of the others have the same latent issue.

Acceptance criteria

• Audit every contract's #[contracterror] pub enum Error for duplicate #[repr(u32)] discriminant values
• Add a lightweight CI check (or a documented convention) preventing this from recurring