feat(neuron): wire candle harness load/unload via GGUF

Stage 2 of the candle-native pivot. Fleshes out CandleHarness with a
LoadedModel registry keyed by model_id, hf-hub-backed GGUF download,
and Qwen3 quantized weight construction via candle-transformers'
quantized_qwen3 module. unload_model drops the entry; Drop on the
candle ModelWeights frees device memory.

Device selection prefers CUDA (gated behind the new `cuda` feature),
falling back to CPU when CUDA is unavailable so default builds work
on non-GPU hosts. The candle CUDA toolchain isn't pulled in unless
`--features cuda` is passed, keeping CI green on CPU runners.

Config gains a [harness.candle] block with an optional hf_cache path.
HarnessRegistry::from_configs now takes HarnessSettings so per-harness
config flows through.

A gated tests/candle_lifecycle.rs exercises real load → list → unload
→ list-empty when run with `--features cuda-integration` against a
host with HF network access. The default-feature test in tests/api.rs
covers the wrong-harness rejection path without needing the network.

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

crates/neuron/src/config.rs

@@ -6,7 +6,7 @@ use figment::{
     providers::{Env, Format, Toml},
 };
 use serde::{Deserialize, Serialize};
-use std::path::Path;
+use std::path::{Path, PathBuf};
 
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct NeuronConfig {
@@ -14,6 +14,25 @@ pub struct NeuronConfig {
     pub port: u16,
     #[serde(default)]
     pub harnesses: Vec<HarnessConfig>,
+    /// Per-harness configuration. Currently only `candle` is recognised.
+    #[serde(default)]
+    pub harness: HarnessSettings,
+}
+
+/// Settings for individual harness implementations. Each harness owns
+/// its own sub-table so users only configure the harnesses they enable.
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct HarnessSettings {
+    #[serde(default)]
+    pub candle: CandleHarnessConfig,
+}
+
+#[derive(Debug, Clone, Default, Serialize, Deserialize)]
+pub struct CandleHarnessConfig {
+    /// HuggingFace cache directory for model weights.
+    /// When unset, defers to hf-hub's default (~/.cache/huggingface).
+    #[serde(default)]
+    pub hf_cache: Option<PathBuf>,
 }
 
 fn default_port() -> u16 {
@@ -35,6 +54,7 @@ impl Default for NeuronConfig {
         Self {
             port: 13131,
             harnesses: vec![],
+            harness: HarnessSettings::default(),
         }
     }
 }