AUS-1016: embedded-host docs bundle and samples/EmbeddedHost

[Astn]

Docs bundle from the API usability review, plus the sample it points at. No library code changes.

samples/EmbeddedHost (in the solution, not packed): the core hosted without ASP.NET Core.

• HttpListenerHost.cs: HTTP through System.Net.HttpListener; body limit, a deadline the host arms itself (504 when it fires), 204 for a notification, 405 for anything but POST.
• StreamHost.cs: any Stream (a named pipe here) with newline-delimited documents both ways, framed with JsonFramer, a bounded buffer, documents run one at a time in order, and a separate write loop fed by a queue so pipelined clients cannot deadlock. Server-side events go out as notifications on the same queue. The -1 from FindDocumentEnd is disambiguated so garbage answers -32700 and closes instead of stalling.
• UiThread.cs: a SynchronizationContext over one thread; every document and every continuation inside a method runs there.
• dotnet run -- check runs all three in process against a client; twelve checks, exit code 0. Run locally: all pass.

README

• Sessions: a guest in someone else's process (plugin, add-in, library) names a session of its own; the reason is spelled out.
• Defining methods: unknown named members are refused with -32602, never dropped. New "Reading what is bound" paragraph for MetaData.Services / RpcMethod / dele.
• Hosting: the framer's -1 semantics and buffer bounding; a new "Embedded" subsection walking the three sample files; the raw-connection paragraph now says replies carry no newline and no Content-Length (same in the AspNetCore README).
• Errors: the -32000 examples become application codes (1000, 1001) with the reason: the specification reserves -32768..-32000.
• Cancellation: the token parameter may sit anywhere in the signature, so (int id, [JsonRpcCancellation] CancellationToken token, int limit = 10) is the natural shape.
• Context: the adapter pattern (pass the transport object, read it before the first await, cast to an interface to stay transport-neutral).
• Installation and Versioning: trimming/AOT is the one constraint that keeps a host off 2.0 until the 2.8 generator.

Site: the sample README is a page ("Embedded host sample", under More); local Typst build reports no missing anchors or files. CHANGELOG has an Added entry.

Linear: AUS-1016.