fix(prefill): resolve promoted final snapshots directly

[FluffyAIcode]

Summary

• resolve the longest available chained snapshot even when intermediate checkpoint entries are absent
• let Primary hot promotion store only the final complete snapshot instead of duplicating every boundary
• retain full-prefix integrity because each chained hash commits all preceding token blocks
• update ADR/runbook semantics and add sparse promoted lookup regression coverage

Live root cause

Remote import promoted the final 217-token snapshot to Primary, but the next request still queried allens because local lookup required boundary 1..N to all exist. The final chained snapshot is independently sufficient and safe.

Tests

• focused cache/service/orchestrator suite — 32 passed
• full platform-neutral suite — 838 passed, 1 skipped
• diff checks pass

Made with Cursor

[cursor]

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