feat(api): POST /api/topologies/compare (provider-backed)

Run a task across topologies via the API and return a leaderboard + quality/cost
Pareto front. The handler builds a ProviderExecutor + JudgeScorer over the
runtime's configured provider/model, then calls cm_orchestrator::compare.

- cm-runtime: expose provider()/model()/max_tokens() accessors on Runtime.
- cm-api: depend on cm-orchestrator (provider feature); add the compare route.
- Integration test runs a 2-topology comparison through the real server
  (scripted provider) → 200 with results + leaderboard. 4 topology tests green;
  offline build + clippy clean.

The topology endpoints (catalog/classify/build/compare) ship to gw-04 with the
upcoming ReactFlow UI in one server+frontend redeploy.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

crates/cm-api/src/routes/topology.rs

@@ -3,11 +3,13 @@
 //! back the topology builder UI. Running/comparing topologies is a later,
 //! provider-backed endpoint.
 
+use axum::extract::State;
 use axum::Json;
+use cm_orchestrator::{compare, Comparison, JudgeScorer, ProviderExecutor};
 use cm_topology::{build, classify, heuristics, Classification, TopologyGraph, TopologyKind};
 use serde::{Deserialize, Serialize};
 
-use crate::{ApiError, Authed};
+use crate::{ApiError, AppState, Authed};
 
 /// A role and its suggested share of the team.
 #[derive(Serialize)]
@@ -70,3 +72,29 @@ pub async fn build_graph(
     let graph = build(req.kind, &roles).map_err(|_| ApiError::BadRequest)?;
     Ok(Json(graph))
 }
+
+/// Request body for running a multi-topology comparison.
+#[derive(Deserialize)]
+pub struct CompareRequest {
+    pub task: String,
+    pub graphs: Vec<TopologyGraph>,
+}
+
+/// `POST /api/topologies/compare` — run a task across the given topologies and
+/// return a leaderboard + quality/cost Pareto front. Uses the configured
+/// provider for both execution (tool-free turns) and the LLM judge.
+pub async fn compare_topologies(
+    State(state): State<AppState>,
+    _auth: Authed,
+    Json(req): Json<CompareRequest>,
+) -> Result<Json<Comparison>, ApiError> {
+    let provider = state.runtime.provider();
+    let model = state.runtime.model().to_string();
+    let executor =
+        ProviderExecutor::new(provider.clone(), model.clone(), state.runtime.max_tokens());
+    let scorer = JudgeScorer::new(provider, model, 16);
+    let cmp = compare(&req.graphs, &req.task, &executor, &scorer)
+        .await
+        .map_err(|_| ApiError::Internal)?;
+    Ok(Json(cmp))
+}