fix(quick-1): bail on NULL KV cache and improve prefill error diagnostics

- engine.rs: new_session() now returns Err when llama_get_memory returns NULL
  instead of silently continuing, preventing decode corruption on 2nd request
- session.rs: decode_prefill error includes token count for easier diagnosis

crates/inference-core/src/session.rs

@@ -52,18 +52,6 @@ struct PrefillChunkPlan {
 unsafe impl Send for GenerationSession {}
 
 impl GenerationSession {
-    /// Tokenize prompt and run prefill decode. Returns a session ready to generate.
-    pub fn new(
-        _lib: Arc<LlamaLib>,
-        _ctx: *mut llama_context,
-        _sampler_cfg: SamplerConfig,
-    ) -> Result<Self> {
-        // Get vocab from context's model (via a workaround: we pass vocab separately)
-        // For now we'll get it indirectly — see Engine::new_session which passes vocab.
-        // This constructor signature will be refined; use the vocab variant below.
-        bail!("Use GenerationSession::with_vocab() instead")
-    }
-
     /// Full constructor called by InferenceEngine::new_session().
     pub fn with_vocab(
         lib: Arc<LlamaLib>,
@@ -159,8 +147,10 @@ impl GenerationSession {
         let prefill_ms = prefill_start.elapsed().as_secs_f64() * 1000.0;
         let prefill_tps = n_prompt_tokens as f64 / (prefill_ms / 1000.0);
         info!(
-            "Prefill finished in {:.2} ms ({:.2} t/s)",
-            prefill_ms, prefill_tps
+            n_prompt_tokens,
+            prefill_ms  = format_args!("{:.2}", prefill_ms),
+            prefill_tps = format_args!("{:.2}", prefill_tps),
+            "session.prefill_complete"
         );
 
         // create session state
@@ -224,6 +214,14 @@ impl GenerationSession {
             self.n_decoded += 1;
             self.tokens_generated += 1;
 
+            if self.tokens_generated % 100 == 0 {
+                tracing::debug!(
+                    tokens_generated = self.tokens_generated,
+                    max_tokens       = self.max_tokens,
+                    "session.generation_progress"
+                );
+            }
+
             // Detokenize
             let token_text = self.detokenize_token(new_token);
 
@@ -249,14 +247,18 @@ impl GenerationSession {
                 self.tokens_per_sec = self.tokens_generated as f64 / elapsed;
             }
         }
+        let stop_reason = if self.tokens_generated >= self.max_tokens {
+            "max_tokens"
+        } else {
+            "eos_or_stop_sequence"
+        };
         info!(
             tokens_generated = self.tokens_generated,
-            tps = format!("{:.1}", self.tokens_per_sec),
-            prefill_ms = self
-                .prefill_ms
-                .map(|v| format!("{:.1}", v))
-                .unwrap_or_default(),
-            "Generation complete"
+            max_tokens       = self.max_tokens,
+            tps              = self.tokens_per_sec,
+            prefill_ms       = self.prefill_ms.unwrap_or(0.0),
+            stop_reason,
+            "session.generation_complete"
         );
     }
 
@@ -347,9 +349,10 @@ fn decode_prefill(
 
         if ret != 0 {
             bail!(
-                "llama_decode (prefill) returned {} while decoding prompt chunk {:?}",
+                "llama_decode (prefill) returned {} for chunk {:?} — likely KV cache not cleared or context window ({} tokens) exceeded by prompt",
                 ret,
-                plan.range
+                plan.range,
+                tokens.len()
             );
         }
     }