Write the C-ABI embedding guide and examples

Summary

Document embedding through the versioned libgoccia dynamic library and stable C ABI, with a worked example for every validated host language.

Why

docs/embedding.md covers FreePascal-native embedding only. Foreign hosts need installation, runtime-discovery, ownership, and compatibility guidance specific to a dynamic library.

Expected behavior

• Document platform artifact names and runtime search/install configuration for .dylib, .so, and .dll.
• Publish the C API reference: ABI versioning, lifecycle, value handles, ref/unref, error handling, thread affinity, and execution limits.
• Include a worked C, Rust, Zig, Crystal, Nim, and C# example sourced from #866.
• Explain library/header compatibility and upgrade rules.
• Document redistribution and release-artifact verification.

Scope notes

• Depends on #864, #865, and #866.
• Parent #823.
• Do not describe static linking as a supported embedding route.

Start with docs/embedding.md and the outputs from #864, #865, and #866, since this guide depends on them. Cover the versioned libgoccia artifacts, stable C ABI, runtime and ownership rules, compatibility, redistribution, and verification. Done means the guide includes validated C, Rust, Zig, Crystal, Nim, and C# examples without presenting static linking as supported.