Reseed lost event history from the KV projection on transition

A record whose event log is empty (the EVENTS stream was purged, or
the record predates event-sourcing) but whose projection exists used
to be stranded - replay found nothing, every transition failed with
"unknown item". transition_or_reseed reconstructs a minimal one-event
history from the projection's own current state and retries, making
"zap the whole event stream" a recoverable operation by design instead
of a slow-motion data loss. Reseed races are guarded by the same
expected-sequence-0 CAS create uses.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/aggregates/mod.rs

@@ -240,6 +240,39 @@ pub async fn create(
     })
 }
 
+/// Reconstructs a minimal one-event history for an aggregate whose
+/// event log is empty but whose KV projection still knows its current
+/// state - what makes wiping the EVENTS stream a recoverable
+/// operation rather than one that strands every pre-wipe record in an
+/// untransitionable limbo. `expected_seq: Some(0)` guards two racing
+/// reseeds the same way `create` guards a double submit.
+pub async fn reseed(
+    js: &async_nats::jetstream::Context,
+    schema: &AggregateSchema,
+    id: &str,
+    state: &str,
+    occurred_at_ms: i64,
+) -> Result<(), TransitionError> {
+    let event_type = schema.event_for_state.get(state).ok_or_else(|| {
+        TransitionError::Store(format!(
+            "cannot reseed {id}: state {state:?} is not declared for bucket {:?}",
+            schema.bucket
+        ))
+    })?;
+    append_event(
+        js,
+        &schema.bucket,
+        id,
+        event_type,
+        serde_json::json!({ "reseeded_from_projection": true }),
+        occurred_at_ms,
+        Some(0),
+    )
+    .await
+    .map_err(classify_publish_err)?;
+    Ok(())
+}
+
 /// Loads and replays `id`'s current state, then - if `target` is
 /// actually reachable from it - appends the corresponding event with
 /// the aggregate's just-replayed sequence as the CAS guard.