feat(gateway): surface mid-prewarm models as Loading on /v1/models

The poller now fetches /health alongside /models on each neuron and
stashes the activation snapshot on NodeState. The /v1/models handler
gains a Pass 3 that synthesises Loading locations from each neuron's
activation.in_progress and activation.pending lists, so a catalogued
model that's mid-prewarm surfaces as `status: "loading"` rather than
appearing absent (loaded=false, locations=[]).

Without this, a client polling /v1/models during a beast restart sees
Qwen3.6-27B disappear for the ~5 minutes the q5k load takes, then
reappear. Now it stays visible the whole time with a clear status.

Adds ModelStatus::Loading to cortex-core. The router's per-node priority
loop gets an explicit (no-op) arm: Loading models aren't routable yet,
and falling through to the catalogue cold-load path is the existing
race — no worse than before, but tagged as a known follow-up needing
neuron-side in-flight tracking on /models/load.

New test_poller_captures_activation_from_health exercises the full
round-trip: mock neuron with empty /models but a pre_warming /health
→ poller writes node.activation. Common test helpers gain
spawn_mock_neuron_with_models_and_health and default_health_response.

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

crates/cortex-gateway/src/state.rs

@@ -27,6 +27,7 @@ impl CortexState {
                     lifecycle_cycles: 0,
                     last_poll: None,
                     discovery: None,
+                    activation: None,
                 },
             );
         }