Reconcile sakila-example README app-section with R399 (dead GraphqlEngine/GraphqlResource/AppContext links)

graphitron-sakila-example/README.md still describes the runtime under a "Runnable reference (the app)" section (roughly lines 17-27) as three example-owned files: GraphqlEngine.java, GraphqlResource.java, and AppContext.java. R399 extracted all of that into graphitron-jakarta-rest; the example now ships a single SakilaGraphitronApplication adapter (a GraphitronApplication SPI implementation) and depends on the library for the /graphql resource, the engine, status-code semantics, the /schema endpoint, and the GraphiQL page. The three source links in that section are dead, and the "three files cover the runtime" framing is false.

R416 reconciled only the GraphiQL-specific claims in this README (the /graphiql/ redirect prose and the deleted META-INF/resources/graphiql/ bundle) because those were in its scope; it deliberately left the broader R399 app-section drift alone rather than expand scope. This item is that follow-up: rewrite the app section around SakilaGraphitronApplication + the graphitron-jakarta-rest dependency, fix or remove the dead GraphqlEngine/GraphqlResource/AppContext links, and keep the "why plain JAX-RS" and per-request-context prose accurate to where that logic now lives.