refactor: eagerly persist session events

docs/cordis-catalog/services.md

@@ -904,10 +904,9 @@ abstract locate(meta: SessionHeader): SessionLocation | undefined
 abstract create(meta: SessionHeader): Promise<void>
 
 /**
- * Durably persist a batch of events (called from the write-behind drain at
- * the `session/flush` checkpoint). Honors the append-only and contiguous-seq
- * contracts: the first event's `seq` MUST equal the stored next-seq (after
- * `load` has durably closed any interrupted turn). Rejects non-JSON-
+ * Durably persist a batch of events. Honors the append-only and contiguous-
+ * seq contracts: the first event's `seq` MUST equal the stored next-seq
+ * (after `load` has durably closed any interrupted turn). Rejects non-JSON-
  * serializable `event.data` with an error naming the offending event type.
  * @param id - the session the batch belongs to.
  * @param events - the contiguous batch to persist, in seq order.

docs/core-data-structures/persistence.md

@@ -6,7 +6,7 @@ The seam is a textbook [capability seam](../../.agents/notes/implemented/archite
 
 ## The flush checkpoint
 
-`session/event` is a *synchronous* notification; persistence plugins buffer it (write-behind) until `session/flush`. The loop awaits an ordinary turn's checkpoint before claiming the next queue item; synchronous idle `inject()` schedules its checkpoint without blocking `send()`, and disposal still drains it. A successful flush durably commits the closed turn as one unit; a rejecting flush is reported through `agent/error` and the logger — never as a session event past the closed turn — while the backend keeps its buffered events for the next flush.
+`session/event` is a *synchronous* notification; persistence plugins copy the event into a per-session controller and start an eager write without blocking the producer. Concurrent events share the current batch, and events admitted during that write trigger a follow-up batch. `session/flush` waits until no current or pending batch remains, so the loop still uses it as the ordering and error-observation checkpoint before claiming the next ordinary turn. A rejected eager write retains its events; an explicit flush retries them and reports failure through `agent/error` and the logger, never as a session event past the closed turn. Disposal performs the same final drain.
 
 ## Crash recovery preserves an interrupted turn