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/main.rs

@@ -1,6 +1,6 @@
 use anyhow::Result;
 use clap::Parser;
-use neuron::{api, config::NeuronConfig, discovery, harness::HarnessRegistry, health};
+use neuron::{api, config::NeuronConfig, discovery, harness::HarnessRegistry, health, startup};
 use std::sync::Arc;
 use std::time::Instant;
 use tokio::sync::RwLock;
@@ -55,6 +55,12 @@ async fn main() -> Result<()> {
     discovery_result.harnesses = registry.names();
     let candle = registry.candle();
 
+    // Activation: load default models before binding the listener.
+    // Each load may take tens of seconds to several minutes depending
+    // on model size and HF cache state — keep TimeoutStartSec in the
+    // systemd unit generous enough to cover the slowest entry.
+    startup::load_default_models(&registry, &cfg.default_models).await;
+
     let health_cache = Arc::new(health::HealthCache::new());
     health_cache
         .set_has_gpus(!discovery_result.devices.is_empty())