PriceRouter explain, compare, and tool manifest

Every call to PriceRouter can now explain itself, and you can compare strategies side by side before you commit. Agents get a downloadable tool manifest so they can wire up PriceRouter with no human in the loop.

PriceRouter is a multi-vendor catalog of pricing models behind one API. These three additions make it transparent.

**Explain after run**: every response can include a step-by-step trace of which provider was picked, why, and what each one would have returned

**Compare strategies**: a side-by-side view in the dashboard runs the same input through `cheapest`, `fastest`, `policy`, and the rest, so you can pick the strategy with eyes open

**Clickable catalog**: each provider/model has its own page with description, cost per unit, and a one-click playground

**Tool manifest**: `/api/price-router` returns a manifest that agent frameworks can consume directly, with each built-in named `lastprice/<provider>-<model>` so the provider is unambiguous