Add idea: Dapr-aware GoRouter

[rkoster]

Explores what it would look like for GoRouter to route based on where a unit of durable work lives rather than which instance is free \u2014 i.e. resolving a Dapr actor/workflow ID to the specific app instance that owns it.

Core observation: Dapr's placement table and GoRouter's route table are the same kind of object (logical identity \u2192 backend instances, NATS-disseminated, churn-reactive), and CF already has instance-addressed routing via X-CF-APP-INSTANCE and __VCAP_ID__.

Presents three depths as a progression:

1. Shallow \u2014 daprd resolves placement, sets X-CF-APP-INSTANCE; zero GoRouter changes.
2. Medium \u2014 GoRouter learns placement over NATS and resolves actor IDs from the standard Dapr invocation path; per-route opt-in via RFC-0027 route options. Actor invocation stops requiring a sidecar on the calling side.
3. Deep \u2014 GoRouter's route table becomes the placement table, potentially removing the need for dapr-placement-server on CF.

Includes an honest risk section on the consistency problem (actors are single-threaded and stateful; Dapr uses Raft to prevent double-activation, while GoRouter's route table is eventually consistent), with 'keep the sidecar authoritative' as the safe mitigation.

Builds on RFC-0055 (identity-aware routing) and RFC-0027 (per-route features). Sibling to the dapr-durable-execution-on-cf idea, which leaves actor placement as its one genuinely open question.