PriceRouter OpenAPI contract and feature page

PriceRouter GET and POST endpoints are now fully documented in the OpenAPI spec with typed schemas for capability, request, and response. A dedicated feature page makes the multi-vendor router discoverable from the marketing site.

The PriceRouter surface is now fully represented in the OpenAPI contract:

`GET /api/price-router` documented with `PriceRouterCapability` response schema (providers, builtins, strategies, metering constants)

`POST /api/price-router` documented with typed request/response schemas including function-name resolution and metering output

Seven new component schemas: `PriceRouterCapability`, `PriceRouterStrategy`, `PriceRouterProvider`, `PriceRouterBuiltin`, `PriceRouterMetering`, `PriceRouterRequest`, `PriceRouterResponse`

New `/features/price-router` marketing page with investor-facing copy framing the multi-vendor routing as "OpenRouter for price computation"

SDK generators and agent tool runtimes can now validate against the canonical contract without relying on runtime discovery alone