Separate development from acceptance gate

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Clarifies the pre-release lifecycle so ordinary development cannot be mistaken for an active release-candidate freeze.

• keeps the repository in protocol_status: development now
• defines acceptance_candidate and released lifecycle states
• requires explicit authorization before acceptance begins
• makes the acceptance transition commit the first freeze-eligible candidate
• requires corrective work to return to development after a candidate is invalidated
• aligns README and MIGRATIONS.md with the manifest
• adds deterministic validator checks for lifecycle consistency

This is a development-process cleanup only. It does not start the acceptance gate.