🧩 Middleware: Response headers injection (scaffold)

Introduce a minimal response headers injection scaffold (incremental toward #39).

Scope

• Create a ResponseHeadersOptions class under src/NLWebNet and wire minimal binding
• Add a simple middleware or extend existing NLWebMiddleware to set one sample header when configured
• Add an example to samples/Demo/appsettings.json

Acceptance Criteria

• Running the demo sets at least one header from config

Labels: configuration, middleware, good first issue

Link: #39