Add comprehensive repository architecture documentation

[Copilot]

Repository lacked architectural documentation for onboarding and maintenance planning.

Changes

• Added REPOSITORY_OVERVIEW.md - 350-line technical overview covering:
  • Architecture diagram showing GStreamer → rialto-gstreamer → Rialto SDK → hardware stack
  • 7 crucial areas prioritized by criticality (sink elements, client management, buffer parsing, playback delegates, DRM/EME, utilities, plugin registration)
  • Risk assessment (thread safety, state machines, buffer lifetime, DRM security)
  • Build system, test infrastructure, and CI/CD workflows
  • Integration points and development priorities

Key Insights Documented

Critical Areas:

• GStreamer sink implementations (public API surface)
• MSE/WebAudio client management (core business logic, threading)
• Buffer parsers (codec-specific, DRM metadata extraction)
• DRM/EME handling (security-sensitive)

Risk Zones:

• Thread synchronization across GStreamer/Rialto/OCDM boundaries
• State machine transitions in GStreamerMSEMediaPlayerClient
• Reference counting at GStreamer/C++ smart pointer boundary

Document serves as technical map for code reviews, incident response, and architectural decisions.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.