[RFC] Define point-in-time universe contract and snapshot model

Parent epic

#251

Deliverable

Write an architecture RFC for the point-in-time universe rule contract and snapshot model shared by research, backtests, and live strategy runs.

The RFC should define:

• The versioned rule schema and composable AND/OR filters.
• Observation time, availability time, and effective membership time.
• Daily, weekly, monthly, and explicit N-day refresh semantics.
• Snapshot identity, persistence, provenance, and replay behavior.
• Missing data, provider limits, and exclusion-reason behavior.
• Migration and compatibility expectations for saved universes.

Acceptance criteria

• The proposal includes concrete request and persisted-record examples.
• Look-ahead protections are expressed as testable invariants.
• Research, backtest, and live consumers use one contract.
• Failure and stale-data behavior is explicit.
• Open questions and implementation slices are listed.
• The RFC is submitted as a focused draft pull request.

How to claim

Comment with a short outline and any prior work you plan to reuse. A maintainer will confirm the scope and assign the issue before implementation begins.