FEATURE-readme-refresh: teach the native v3 API in the standalone manual

Replace historical API chapters with a checked C++20 tutorial and guided reference, preserving the manual flow, community and licensing. Document listener/provider distinctions, streaming and ownership, TLS policies, typed capacities, hooks, cancellation and shutdown. Index the fifteen native examples.

Adapt the README checker with observed RED/GREEN mutation regressions and local-anchor checks. Verify strict markdownlint, shared fence checks, all thirteen C++ blocks with matching installed TLS-off/on libraries, shared/static consumer metadata and forced archive linkage, and 66 local HTTP assertions across twelve executions. Nonlocal platform verification remains CI/v3 PR owned.

Author: etr