Specification clarifications for container structure, canonicalization, and validation:
• Clarified that all head.* fields MUST appear exclusively inside the head object (Section 3.2)
• Added rule that absent optional fields MUST NOT be serialized as null (Section 3.4)
• Canonical JSON clarification: array order MUST be preserved and treated as significant (Section 3.7)
• Introduced explicit container verification procedure and validation order (Section 3.10)
No changes to container schema or wire format.
Fully backward-compatible within the 5.0.x series.