chore(neuron/beast): switch default-model quant from q5k to q6k

q5k produced NaN logits on Qwen/Qwen3.6-27B under candle TP=2 (sampler
fell over with "logits unhealthy nan: 248320/248320"). q6k is the
quant that worked well in production under mistral.rs on the same
hardware, so it's the right baseline for verifying the mempool-trim
fix.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

asset/neuron/beast.toml

@@ -19,6 +19,6 @@ name = "candle"
 [[default_models]]
 model_id = "Qwen/Qwen3.6-27B"
 harness = "candle"
-quant = "q5k"
+quant = "q6k"
 tensor_parallel = 2
 devices = [0, 1]