Spike: Segment Membership + Evaluation Streaming integration

### Goal

Evaluation Streaming pushes evaluations into customer DWHs. There may be a path to compute or maintain segment membership directly off that stream — eliminating the separate Identity mirror. Investigate:

- What does the streaming row carry today? Is it sufficient to evaluate membership without re-reading traits?
- Can the engine layer translate to a ClickHouse query over the streaming table without a separate IDENTITIES mirror?
- What's the lag profile, and does it satisfy the V1 freshness target the daily refresh hits today?

### Output

- A user-facing docs PR.
- A scoped implementation issue, or an epic, or close the spike as won't fix.