Audit terraphim_router decisions before any external decision-model router #966

Goal

Before anyone proposes an external decision model (Jev, Levanto Sage) as a router inside ADF, document what terraphim_router already decides and on what signals. We have built routing twice; check what exists before adding a third.

Tasks

1. Inventory the routing decisions terraphim_router makes today: inputs, outputs, decision points, and fallbacks.
2. Read Levanto's switchboard-poc (MIT, https://github.com/levantolabs/switchboard-poc). It is an OpenAI-compatible proxy where one decision-model Choice call picks the cheapest capable model per request. Note any idea worth taking. Do not add it as a dependency.
3. List where a typed Choice/Score call would replace hand-written routing logic, and where it would only add latency and egress.

Acceptance

• A short design note: current decisions, gaps, and candidate insertion points (if any) for a decision-model call, each with its cost and latency impact.

Mirror of Gitea terraphim/terraphim-ai#3419 (source of truth).