Design snapshot fixture import migration path

[flyingrobots]

Problem

HT-0149 quarantines the full-snapshot runtime as a fixture. Existing snapshot roots may still need an import path into graph-backed rope authority without laundering old fake history as real causal evidence.

Acceptance Notes

• Define snapshot-root import as TextBlob plus initial RopeHead/checkpoint.
• Mark prior snapshot history as non-authoritative projection evidence.
• Define production guard behavior during migration.
• Link the policy back to HT-0149 and PR Document graph-backed rope runtime gate #205.