docs: add language-agnostic product and porting specification

Add docs/product-spec.md, a normative, implementation-neutral specification
of the SDK's behavior and contracts. It states the product thesis (an
HTTP-client toolkit rather than a client), the pluggable seams and their
discovery rules, the immutable HTTP model, both execution-pipeline layers,
and the retry, redirect, authentication, pagination, streaming, serialization,
observability, configuration, and transport/async-adapter contracts as
RFC 2119 requirements. Each requirement carries a stable identifier, a
rationale, and a conformance-test note, and the document closes with a
glossary, a conformance checklist, and a consolidated requirement index,
so the SDK can be reimplemented faithfully in another language or runtime.

Author: OmarAlJarrah