feat: version rollout weight updates

Track a monotonic policy version across optimizer steps, scheduler updates, and rollout results. Serialize synchronous generation with weight acknowledgements and invalidate reusable prefix KV entries so cached samples remain attributable to the behavior policy that generated them.

docs/developer/architecture.md

@@ -673,6 +673,7 @@ classDiagram
             +Tensor responses
             +Tensor response_mask
             +Tensor logprobs_old
+            +int policy_version
             +List[str] prompt_texts
             +List[List[str]] response_texts
         }
@@ -695,10 +696,14 @@ classDiagram
             +float top_p
             +float frequency_penalty
             +int rep_window
+            +int policy_version
+            +update_weights(policy_version) int
             +generate(batch) RawRollout
         }
 
         class RolloutRunner {
+            +int policy_version
+            +update_weights(policy_version) int
             +step()
             +clear_cache()
             +__call__(batch) Tuple[RolloutResult, bool]
@@ -854,11 +859,13 @@ classDiagram
             +int max_seq_len
             +str device
             +torch.dtype dtype
+            +int policy_version
             +add_task(prompt, **kwargs) str
             +remove_task(task_id)
             +start()
             +stop()
             +get_stats() Dict
+            +update_weights(policy_version) int
             +run_batch(prompt_ids_list, max_tokens, temperature, top_p, top_k, frequency_penalty, rep_window, return_logprobs) Union[List[List[int]], List[Tuple[List[int], List[float]]]]
         }
 
@@ -871,6 +878,7 @@ classDiagram
             +inc_ref(idx)
             +touch(idx)
             +ref_count(idx) int
+            +clear_cached() int
         }
 
         class RadixNode {
@@ -897,6 +905,7 @@ classDiagram
             +extend(state, pos) bool
             +write_indices(state, prompt_ids)
             +record_hashes(state, prompt_ids, start_logical_page)
+            +invalidate_cache() int
         }
 
         class ContiguousStrategy {
@@ -960,6 +969,7 @@ classDiagram
             +task_extend(task_id, pos) bool
             +task_cached(task_id) int
             +task_record_hashes(task_id, prompt_ids, start_logical_page)
+            +invalidate_cache() int
             +bind(task_ids, workspace) KVCache
         }