fix(mlx): point Qwen 0.6B at the published mlx-community repo

The 0.6B slot was aliased to the 1.7B repo as a temporary fallback
because `mlx-community/Qwen3-TTS-12Hz-0.6B-Base-bf16` wasn't published
when MLX support shipped. That conversion is live now, so use it —
Apple Silicon users picking 0.6B get the actual 0.6B model (1.2 GB
instead of 3.5 GB).

Also drops the now-obsolete troubleshooting entry and updates the
triage notes in PROJECT_STATUS.md.

Fixes #485.

Co-Authored-By: Claude Opus 4.7 (1M context) <[email protected]>

backend/tests/test_qwen_download.py

@@ -175,7 +175,7 @@ async def main():
     print("   ✅ Server is running")
 
     # Test model
-    model_name = "qwen-tts-0.6B"  # Note: 0.6B currently maps to 1.7B on MLX
+    model_name = "qwen-tts-0.6B"
     
     # Check current status
     print(f"\n📊 Checking status of {model_name}...")