v0.9.0 — steelyard umbrella package + serveCommerce

Steelyard v0.9 makes the SDK easy to start with. A new steelyard umbrella package
replaces the six-package install and the 500-plus-symbol surface with one install and
a curated front door: the ~15 symbols most integrators actually need, re-exported from
the existing packages. Nothing is hidden — the underlying @steelyard/* packages are
unchanged and still importable directly for the full surface.

The headline is serveCommerce(manifest): one call serves your catalog over every
read surface — commerce.json, the plain HTTP API, MCP, ACP, and UCP — from a single
manifest, read-only by default with no PSP or keys required. It composes the existing
protocol handlers behind one path router (a generalization of the coffee-shop example),
so behavior is identical to wiring each surface by hand. The getting-started guide now
leads with a build-your-own quickstart: install one package, write eight lines, and
curl a live multi-protocol endpoint of your own catalog in under two minutes. This
release is purely additive; every existing import keeps working.