Routing: unify DirectStream ACK routes and FanoutTree route tokens #600
Description
Parent: #577
Problem
We currently maintain two route-knowledge mechanisms:
- ACK-trace routes (DirectStream)
- Overlay route tokens and root-proxy semantics (FanoutTree)
This duplication increases complexity in routing decisions and makes behavior harder to reason about across targeted vs fanout-backed delivery paths.
Goal
Define a unified route-hint interface that lets send paths select the cheapest reliable route while preserving bounded behavior.
Scope
- Specify a common route-hint abstraction consumable by DirectStream and FanoutTree-backed paths.
- Define fallback order: use proven cheap hints first; otherwise fall back to shard overlay reachability.
- Add observability counters for hint hit/miss/fallback behavior.
- Validate with deterministic sims/tests that delivery correctness and bounded costs are preserved.
Acceptance
- A documented route-hint interface exists with clear fallback semantics.
- At least one targeted + one fanout path use the shared abstraction.
- Metrics demonstrate bounded fallback behavior and no correctness regressions.