Add per-package API reference docs

[nficano]

TS SDK has docs/packages/{client,core,runtime,sdk,bun,express,fastify,hono,node,middleware-otel}.md — one page per package, listing public API surface.

C# SDK has narrative guides numbered 00–18 but no per-package API reference. Add docs/packages/Arcp.{Core,Client,Runtime,AspNetCore,Otel,Hosting,Cli}.md covering each shipped NuGet package's public types, configuration options, and code samples.

Steps:

1. List public types in each project via dotnet build and reflection or by scanning public class/public record/public interface
2. Group by package
3. For each public API, document: signature, purpose, example usage
4. Cross-link to the existing numbered guides where relevant

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Audit reference: ARCP SDK Audit v1.1 (2026-05-19)