feat(cortex-gateway): C3 — propagate vision capabilities through /v1/models

ModelEntry and CortexModelEntry gain a `capabilities: Vec<String>`
field (serde-default for back-compat). The poller copies it verbatim
from each neuron's ModelInfo.capabilities; list_models computes the
union across every node where a model is loaded so a checkpoint loaded
text-only on one neuron and text+vision on another reports both to the
fleet. Catalogue-only and mid-prewarm entries default to empty until
the catalogue gains a capabilities declaration.

Aliases inherit their target's capability union. New gateway test mocks
two nodes with differing capability arrays and asserts the unioned
/v1/models response.

Closes part of #16 (Stage C3).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/cortex-gateway/tests/eviction.rs

@@ -91,6 +91,7 @@ async fn test_evict_lru_model() {
                 status: ModelStatus::Loaded,
                 last_accessed: Some(Utc::now() - chrono::Duration::hours(2)),
                 vram_estimate_mb: Some(8000),
+                capabilities: Vec::new(),
             },
         );
         node.models.insert(
@@ -100,6 +101,7 @@ async fn test_evict_lru_model() {
                 status: ModelStatus::Loaded,
                 last_accessed: Some(Utc::now()),
                 vram_estimate_mb: Some(8000),
+                capabilities: Vec::new(),
             },
         );
     }
@@ -163,6 +165,7 @@ async fn test_eviction_increments_lifecycle_cycles() {
                 status: ModelStatus::Loaded,
                 last_accessed: None,
                 vram_estimate_mb: None,
+                capabilities: Vec::new(),
             },
         );
     }