Initial commit: project scaffold and kernel contract v0

Lay out the design that everything else builds on: a small frozen
kernel plugins treat as a shared language, the plugin contract, the
two-assembly hot-reload model, and the build order through M4.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01N1qPfzq8TDCUMFMV3UwV5N

.gitignore

@@ -0,0 +1,19 @@
+## .NET
+bin/
+obj/
+*.user
+*.suo
+.vs/
+
+## Rider / VS Code
+.idea/
+.vscode/
+*.DotSettings.user
+
+## OS
+.DS_Store
+Thumbs.db
+
+## Build output
+out/
+artifacts/