docs: record the collector batch size defect

docs/evidence.md

@@ -99,6 +99,24 @@ ConfigMap, which the kube-prometheus-stack Grafana sidecar loads as
 `nereus.yaml`. The correlation requires the API at `LOG_LEVEL=info`, because the
 per-request log line is the only one that carries `trace_id`.
 
+Two collector defects were found and fixed on the way to this result, and both
+belong in the report as corrective actions. The first is the capability problem
+above. The second only appears after a restart: `start_at: beginning` makes the
+receiver replay whole log files, the batch processor had no size cap, and Loki
+refused the resulting push.
+
+```text
+HTTP 503, ResourceExhausted: grpc: received message larger than max
+(4378796 vs. 4194304)
+```
+
+The exporter retries that request unchanged, so the backlog never drains.
+Capping the batch with `send_batch_size: 1000` and `send_batch_max_size: 2000`
+keeps every push under Loki's 4 MiB gRPC limit. What remains after a restart is
+a `Permanent` rejection of replayed lines older than Loki's acceptance window,
+which drops those lines rather than blocking, and leaves live traffic
+unaffected.
+
 ## Presentation commands
 
 ```bash