feat: expose headless Runtime bindings entrypoint

[artiphishle]

Closes #22

Summary

• add public @ankhorage/runtime/bindings package entrypoint;
• expose the existing canonical Runtime binding resolution and API operation executor surface through that headless entrypoint;
• keep the root Runtime API unchanged for Expo/React Native consumers;
• add regression coverage that the headless entrypoint contains no Renderer/RuntimeScreen/React Native imports;
• add a minor changeset for the new public subpath.

Why

Studio API7 needs to execute the released canonical Runtime -> Data Sources Nutrition path in a Bun/Node acceptance test. Runtime 2.0.0 exposes these helpers only through the root entrypoint, which also eagerly exports React Native renderer modules. A public headless entrypoint preserves package ownership without deep imports, copies, mocks, or compatibility paths.

Scope

No binding semantics change. No API execution duplication. No renderer restructuring. No internal API generation.

Release boundary

Merge and release Runtime before Studio #298 consumes @ankhorage/runtime/bindings.