refactor(neuron): cut mistralrs/llamacpp, scaffold candle harness

Stage 1 of the candle-native pivot. Replaces the external-process
harness model (mistralrs over HTTP, llamacpp placeholder) with an
in-process Harness trait whose sole implementation is candle. The
trait keeps its shape so future engines slot in additively, but
start/stop default to no-ops and HarnessConfig drops endpoint and
systemd_unit since no harness needs external supervision.

Behaviour is unchanged on the wire: load_model returns a "not
implemented yet (Stage 2)" error and list_models is empty. The
gateway-side proxy, poller, and router are untouched.

CLAUDE.md Phase 11 (llama.cpp) and Phase 12 (mistral.rs COPR) are
marked superseded; the staged plan lives in
~/.claude/plans/create-a-more-aggressive-calm-naur.md.

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

crates/cortex-gateway/tests/streaming.rs

@@ -51,18 +51,18 @@ async fn test_streaming_sse_passthrough() {
     }
 
     assert!(
-        chunks.len() >= chunk_count + 1,
-        "expected at least {} chunks (got {}): {:?}",
-        chunk_count + 1,
+        chunks.len() > chunk_count,
+        "expected more than {} chunks (got {}): {:?}",
+        chunk_count,
         chunks.len(),
         chunks,
     );
 
     assert_eq!(chunks.last().unwrap(), "[DONE]");
 
-    for i in 0..chunk_count {
+    for (i, chunk) in chunks.iter().enumerate().take(chunk_count) {
         let chunk_json: serde_json::Value =
-            serde_json::from_str(&chunks[i]).expect("chunk should be valid JSON");
+            serde_json::from_str(chunk).expect("chunk should be valid JSON");
         assert_eq!(
             chunk_json["choices"][0]["delta"]["content"],
             format!("token{i}")