docs: add ARCHITECTURE.md

[melmallow]

Summary

Adds a deep architecture document at the repo root covering how Rioc is built (companion to README.md, which covers how to run it).

The doc walks through:

• The two-vLLM split (audit on :8100, conversation on :8101 with audio modality + fallback)
• Module responsibilities, including the trap that cloud_brain.py and vision_test.py are not loaded at runtime
• Background tasks and the locks/shared-state surface for the single-process model
• The three FRAME_SOURCE modes (webhook, local_yolo, live_ffmpeg stub) and the PresenceLock throttling
• The 8-step cloud_audit_loop vision pipeline
• ConversationManager state machine, the _TURN_ROLE_REMINDER re-anchoring, and _speaking_event half-duplex sync
• All five speaker-delivery paths and the audio-format gauntlet
• SQLite schema, audio_logs/ retention, env-var contract
• Concurrency model + 13 known sharp edges

Test plan

• Skim ARCHITECTURE.md for correctness against current main.py / conversation_manager.py
• Confirm the "Sharp edges" list reflects real, current behavior (not stale)

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