docs: define Fold-era sovereignty and chronology

[colorpulse6]

Summary

• establishes an absolute sovereign interior that Fold connection cannot expose without willing consent
• distinguishes bleed, communion, intrusion, and public memory, with durable refusal and alternate critical-information paths
• separates entering the campaign's Fold region from the optional late-game choice to bind a mature civilization to the Fold
• protects a substantial frontier → living civilization → Legacy Civilization progression before any mutation, shared-memory UI, or next-cycle war
• frames Fold religions as plural post-Threshold world actors, with only uncertain cults and philosophies beforehand
• keeps M1–M6 explicitly pre-Fold-transformation

Why

Sector Zero's late-game recursion only has weight if the player first builds a civilization worth transforming and defending. The Fold can reveal connectedness without erasing personhood, collapsing the civilization game into a prerequisite meter, or leaking far-future systems into the active roadmap.

Verification

• independent spec review approved the chronology and religion boundaries after one scope-gate correction
• earlier memory-sovereignty review clarifications incorporated for forced fusion and posthumous authorization
• git diff --check
• branch pushed at 2746854
• PR is mergeable against main
• docs-only change; no runtime code or assets modified

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