Add architecture, planning, and conventions documents

Initial documentation for the multi-peer video routing system:
- architecture.md covers graph model, transport protocol, relay design,
  and the Pi get-it-on-the-wire-first rationale
- planning.md defines module build order and directory structure
- conventions.md captures C11 code style, naming, error handling approach,
  and directory layout rules

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1 @@
+reference/