feat: add fluid alliance affordances

[christopherjnelson]

Summary

• remove presentation-driven alliance size and active-alliance capacity limits
• allow unaffiliated agents to request entry from allied recipients while preserving authoritative proposal attribution
• expose exact frozen diplomacy affordances and bounded Patient Zero feasibility data
• advance prompt attribution to text-flat-json-v5 while retaining v3/v4 archive compatibility
• document the contract in ADR 0016 and update architecture, security, gameplay, and testing guidance

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