feat: show per-step OpenRouter cost on the Trajectory tab

Extend the openRouterCost projection with a per-step priced-cost map
keyed by turn:step so surfaces can render spend without re-pricing, and
have the Trajectory view read it through the framework useProjection seat:
each priced assistant step shows its USD cost on the request boundary chip
and in the inspector summary, while unpriced steps stay blank. Also fix a
pre-existing exactOptionalPropertyTypes error in extractCacheRates that the
host typecheck surfaced.

packages/client/ui-trajectory/tsconfig.json

@@ -32,6 +32,9 @@
     {
       "path": "../../compaction/compaction"
     },
+    {
+      "path": "../../llm/openrouter-usage"
+    },
     {
       "path": "../../runtime-diagnostics/invariants"
     }