- Format: {proposal_code}:E{seq:05x} (e.g. PROJ01:P00001:E00001)
- Prefix 'E' for Essential, 5-digit zero-padded hex sequence
- Sequence scoped per Proposal, derived from max existing code
- No separate counter table needed (uses max-suffix approach)
- Supports batch_offset for bulk creation during Proposal Accept
- Includes validate_essential_code() helper