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/mod.rs

@@ -2,8 +2,10 @@
 
 pub mod assemble;
 pub mod chat;
+pub mod gate;
 pub mod info;
 pub mod request;
+pub mod retry;
 
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub enum ChatMessage {
@@ -136,11 +138,15 @@ impl std::error::Error for InferError {}
 
 pub struct Client {
     pub(crate) cfg: crate::config::Config,
+    pub(crate) gate: gate::SlotGate,
 }
 
 impl Client {
     pub fn new(cfg: crate::config::Config) -> Self {
-        Self { cfg }
+        Self {
+            cfg,
+            gate: gate::SlotGate::new(),
+        }
     }
     pub fn config(&self) -> &crate::config::Config {
         &self.cfg