adk-2.0 consumer: agent_transfer_chains view

Parent tracker: #190 (v17 contract).
Wave: 5 — Consumer views.
Blocked by: #195 (`attributes.adk.app_name` identity stamping — grouped on), #200 (producer AGENT_TRANSFER), #210 (cross-event deployment plumbing).

## Contract

Ordered transfer graph per `(JSON_VALUE(attributes, '$.adk.app_name'), user_id, session_id, invocation_id)`, sourced from `AGENT_TRANSFER` events. Full telemetry identity required (multi-tenant table).

## Acceptance

- [ ] One row per transfer with `(from_agent, to_agent, timestamp, ...identity)`.
- [ ] Multi-agent fixture exercises a chain of 3+ transfers and verifies ordering.
- [ ] Null-safe for rows without `attributes.adk` (pre-2.0).

## References

- #190 (v17 consumer view 2).