feat(neuron): load default_models on service activation

Stage 5 of the candle-native pivot. Adds first-class support for
auto-loading a configured set of models when the neuron service
activates.

Config:
- NeuronConfig.default_models: Vec<ModelSpec> (defaults to []).
- neuron.example.toml ships a commented [[default_models]] example.

Activation flow (crates/neuron/src/startup.rs::load_default_models):
- Sequential — VRAM contention makes parallel loads risky.
- Per-entry timing logged at info level on success.
- Failures logged as warnings; the next entry is still attempted.
- An empty list short-circuits without log noise.

Called from main.rs after the registry is built and before the axum
listener binds, so /models reflects the loaded state from the very
first request.

data/neuron.service gains TimeoutStartSec=1800s. With activation
blocked on potentially slow first-time HF downloads + GGUF
materialisation, systemd's default 90s would kill larger model loads
mid-flight.

Two non-gated tests in tests/activation.rs cover the
continues-past-failure and empty-list paths using a synthetically
unknown harness name to fail loads fast without touching the network.
The cuda-integration test from earlier stages still exercises the
real load/unload lifecycle.

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

crates/neuron/src/config.rs

@@ -1,6 +1,6 @@
 //! Neuron configuration loaded from neuron.toml.
 
-use cortex_core::harness::HarnessConfig;
+use cortex_core::harness::{HarnessConfig, ModelSpec};
 use figment::{
     Figment,
     providers::{Env, Format, Toml},
@@ -17,6 +17,12 @@ pub struct NeuronConfig {
     /// Per-harness configuration. Currently only `candle` is recognised.
     #[serde(default)]
     pub harness: HarnessSettings,
+    /// Models to auto-load when the neuron service activates. Each entry
+    /// is loaded sequentially before the HTTP listener binds. A failure
+    /// on any single entry logs a warning and proceeds — broken entries
+    /// don't prevent the rest of the fleet from starting.
+    #[serde(default)]
+    pub default_models: Vec<ModelSpec>,
 }
 
 /// Settings for individual harness implementations. Each harness owns
@@ -55,6 +61,7 @@ impl Default for NeuronConfig {
             port: 13131,
             harnesses: vec![],
             harness: HarnessSettings::default(),
+            default_models: vec![],
         }
     }
 }