Add the per-slot gate and chat_with_retry

Implemented SlotGate (one request in flight per server slot, FIFO queue,
bounded) and chat_with_retry (retry only server-gone errors, jittered
backoff within a window and attempt budget). chat acquires the gate and
maps GateFull to Busy; Client gained a gate field.

Implemented-By: OpenCode session (model recorded in docs/implementer-log.md)

crates/loopd/src/llama/chat.rs

@@ -35,6 +35,15 @@ impl Client {
         req: &ChatRequest,
         on_event: &mut dyn FnMut(&ChatEvent),
     ) -> Result<Completion, InferError> {
+        // The gate holds one request per slot from before it is sent until chat returns; a second
+        // request on the same slot queues here, bounded, and is told (Queued) or refused (Busy).
+        let _permit = self
+            .gate
+            .acquire(req.slot, self.cfg.limits.queue_len, &mut |ahead| {
+                on_event(&ChatEvent::Queued { ahead })
+            })
+            .map_err(|_| InferError::Busy)?;
+
         // 1. Send.
         let body_text =
             build_body(&self.cfg, req).map_err(|e| InferError::Protocol(e.to_string()))?;