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/llm/openrouter-usage/README.md

@@ -54,7 +54,10 @@ step) against the pricing table. Attribution prefers the assembled message's
 own `provider`/`model`; a chunk-only (failed) step prices from the newest
 `request/context` route. A step on a non-`openrouter` provider is outside the
 domain and changes nothing; an OpenRouter step whose model has no pricing
-entry counts as an unknown (unpriced) step.
+entry counts as an unknown (unpriced) step. The projection's `steps` view
+field maps each priced step to its cost in USD under `${turn}:${step}` keys,
+so per-step surfaces can render spend without re-pricing; unpriced steps are
+absent from the map.
 
 ## Extension points