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.

pnpm-lock.yaml

@@ -2969,6 +2969,9 @@ importers:
       '@deepseek-ai/dsh-invariants':
         specifier: workspace:^
         version: link:../../runtime-diagnostics/invariants
+      '@deepseek-ai/dsh-openrouter-usage':
+        specifier: workspace:^
+        version: link:../../llm/openrouter-usage
       '@deepseek-ai/dsh-tools':
         specifier: workspace:^
         version: link:../../core/tools