docs: media player subsystem architecture and conventions

Document the media server process, /media/ port map, library layout on
the ./media bind mount, Hope's media MCP toolset, the music/mic
mutual-exclusion rule, and yt-dlp download/proxy conventions.

tests/README.md

@@ -27,6 +27,7 @@ python -m pytest tests/ -v -m "not slow"   # skip LLM/network-heavy tests
 | `test_web_ui.py` | nginx HTTPS UI (:9178) + token endpoint (:8091) | static assets, `/token` (direct + via nginx), `/livekit/` proxy |
 | `test_llm_api.py` | LLM API directly | completion, tool calling, `enable_thinking=false` latency, streaming |
 | `test_mcp_tools.py` | MCP servers via `docker exec` | weather/time, authenticated remote memory save/recall/delete, and local skills — probes are cleaned up |
+| `test_media.py` | media server API + player UI + media MCP | library/player state flow, Range proxy, static mp3 Range, live YT Music search (`-m slow`), gated full download (`HOPE_RUN_YTDLP_DOWNLOAD=1`) |
 | `test_agent_integration.py` | running container | agent process alive, worker registered with LiveKit, all 4 supervised processes up, container health, LiveKit HTTP API |
 | `test_compaction.py` | `HopeLLM._compact_context` in-container | compaction triggers >24 items, system prompt preserved, short context untouched |