feat(neuron): bind listener before pre-warm, surface activation in /health

Two coupled changes addressing the 2026-05-26 validate-neuron failure
where a fresh deploy of beast had /health unreachable for ~5 minutes
while Qwen3.6-27B q5k materialised, even though systemd reported the
unit as active.

1. main.rs no longer awaits load_default_models before binding axum.
   The listener binds first; pre-warm runs in a spawned background
   task that holds a read lock on the harness registry for the
   duration of its sequential load loop. Concurrent on-demand
   /models/load and /v1/chat/completions traffic still flow.

2. /health gains an `activation` field carrying:
     state         pre_warming | ready
     pending       model ids queued but not started
     in_progress   model id currently loading (Option)
     completed     model ids loaded successfully this activation
     failed        [{model_id, error}] for failed entries
   The field is `#[serde(default)]` so a pre-change cortex polling a
   new neuron — or vice versa — keeps working.

`ActivationTracker` (new module `neuron::activation`) owns the
RwLock-wrapped state; load_default_models takes a tracker reference
and updates it per-model. NeuronState holds an Arc clone for the
/health handler.

Tests updated to construct trackers and assert state transitions
(empty noop, two failures → ready with both in `failed`).

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

crates/neuron/tests/activation.rs

@@ -2,7 +2,9 @@
 //! individual failures so a single broken catalogue entry doesn't
 //! prevent the rest of the fleet from starting.
 
+use cortex_core::discovery::ActivationState;
 use cortex_core::harness::{HarnessConfig, ModelSpec};
+use neuron::activation::ActivationTracker;
 use neuron::config::HarnessSettings;
 use neuron::harness::HarnessRegistry;
 use neuron::startup;
@@ -37,7 +39,8 @@ async fn test_load_default_models_skips_unknown_harness() {
         },
     ];
 
-    startup::load_default_models(&registry, &specs).await;
+    let activation = ActivationTracker::new(&specs);
+    startup::load_default_models(&registry, &specs, &activation).await;
 
     let listed = registry
         .list_all_models()
@@ -47,10 +50,28 @@ async fn test_load_default_models_skips_unknown_harness() {
         listed.is_empty(),
         "no models should be loaded after failed entries"
     );
+
+    // Both specs should land in `failed`; tracker should flip to ready.
+    let snapshot = activation.snapshot().await;
+    assert_eq!(snapshot.state, ActivationState::Ready);
+    assert!(snapshot.pending.is_empty());
+    assert!(snapshot.in_progress.is_none());
+    assert!(snapshot.completed.is_empty());
+    assert_eq!(snapshot.failed.len(), 2);
+    let failed_ids: Vec<&str> = snapshot
+        .failed
+        .iter()
+        .map(|f| f.model_id.as_str())
+        .collect();
+    assert!(failed_ids.contains(&"model-a"));
+    assert!(failed_ids.contains(&"model-b"));
 }
 
 #[tokio::test]
 async fn test_load_default_models_empty_is_noop() {
     let registry = HarnessRegistry::new();
-    startup::load_default_models(&registry, &[]).await;
+    let activation = ActivationTracker::new(&[]);
+    startup::load_default_models(&registry, &[], &activation).await;
+    let snapshot = activation.snapshot().await;
+    assert_eq!(snapshot.state, ActivationState::Ready);
 }