No guard compiles the README and guide C# snippets #388

Nothing in the repository compiles the C# snippets in README.md or docs/guide/*.md.

• tests/OpenCode.Sdk.Tools.Tests/Documentation/ReadmeGuideTableTests.cs checks only that the README's guide table lists every guide.
• tests/OpenCode.Sdk.Tools.Tests/Packaging/PackedReadmeTests.cs checks the packed README's presence and links, not its code.
• docs/guide/README.md asks authors to compile a snippet before adding it, but nothing checks that afterwards.

Why it matters: a public signature change can leave every snippet stale while CI stays green. FRZ-H2b is about to change four door signatures (streams and traversals gain requestOptions before the cancellation token). That breaks 11 positional-token snippets across streaming.md, getting-started.md and connection-modes.md, plus README.md:186. Those edits will be made and checked by hand. The next signature change won't have that luck.

Shape of a fix (for whoever owns docs tooling): extract fenced csharp blocks from README.md and docs/guide/*.md and compile them against the SDK in a Tools.Tests guard. A block that is deliberately a fragment carries an explicit marker. Found while planning FRZ-H2b (faz4/FRZ/gen-frz-h2b-doors.plan.md, amendment item 1).