feat(processor): support the ServiceLoader static provider() method

[LMLiam]

Problem

java.util.ServiceLoader (Java 9+) supports two provider shapes: a public no‑arg constructor, or a public static provider() factory method returning the service type. The processor implicitly assumes the constructor form and offers no way to register a provider that exposes a static provider() method, even though such a class is a valid SPI provider.

Evidence

• Generation writes the provider's binary name into META-INF/services/<contract> (ServiceSchemeProcessor.kt:156-170) with no awareness of provider methods.
• Discovery is purely annotation‑driven on the class (ServiceSchemeProcessor.kt:48-63); there is no recognition of a static provider() factory.
• README only shows the constructor form (README.md:71-92).

Proposed implementation

• Detect a public static <ContractType> provider() method on annotated providers and treat it as a valid registration (the service‑file entry stays the provider class name; ServiceLoader handles the factory automatically).
• Combine with provider‑instantiability validation (separate issue): a provider is valid if it has either a public no‑arg constructor or a public static provider() method.
• Document the factory‑method option in the README.

Acceptance criteria

• A provider exposing only a public static provider() method is accepted and loadable at runtime via ServiceLoader.
• Tests cover the provider‑method form (Java + Kotlin @JvmStatic).