feat(api-playground): a Mercury-branded page around Swagger UI on the Reactive HTTP server - the Spring Boot dependency is gone

[acn-ericlaw]

What

• extensions/api-playground depends on platform-core instead of rest-spring-4; REST automation serves the page and /api/specs on port 8200; api.playground.apps is the optional folder of a developer’s own documents; the example demo.yaml targets port 8200.
• A Mercury-branded index.html with its stylesheet and script: URL field, MiniGraph dialog (host, port, graph ID or session ID), Bundled list, file picker and drag-and-drop. The fetched Swagger UI assets move to the ignored public/swagger-ui subfolder; the fetch scripts, ignore rule and enforcer path follow.
• README rewritten; the graph contract guide’s round trip uses the dialog; CHANGELOG Unreleased items 3 and 4.

Why

• Upstream’s page made the user type the engine URL into an explore bar. The dialog turns the graph contract’s OpenAPI endpoint into a one-step load, and a dropped file covers the exported-document case. Dropping Spring removes a dependency the Reactive HTTP server already covers, and leaves one port and one origin, so Try it out works on the application’s own example.

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