test: Single-partition test confirms broker data retrieval bug

Phase 8: Single Partition Test - Isolates Root Cause

Test Configuration:
  - 1 topic, 1 partition (loadtest-topic-0[0])
  - 1 producer (50 msg/sec)
  - 1 consumer
  - Duration: 2 minutes

Results:
  - Produced: 6100 messages (offsets 0-6099)
  - Consumed: 301 messages (offsets 0-300)
  - Missing: 5799 messages (95.1% loss!)
  - Duplicates: 0 (no duplication)

Key Findings:
  ✅ Consumer stops cleanly at offset 300
  ✅ No gaps in consumed data (0-300 all present)
  ❌ Broker returns 0 messages for offset 301
  ❌ HWM shows 5601, meaning 5300 messages available
  ❌ Gateway logs: "CRITICAL BUG: Broker returned 0 messages"

ROOT CAUSE CONFIRMED:
  - This is NOT a buffer flush bug (unit tests passed)
  - This is NOT a rebalancing issue (single consumer)
  - This is NOT a duplication issue (0 duplicates)
  - This IS a broker data retrieval bug at offset 301

The broker's ReadMessagesAtOffset or FetchMessage RPC
fails to return data that exists on disk/memory.

Next: Debug broker's ReadMessagesAtOffset for offset 301

test/kafka/kafka-client-loadtest/single-partition-test.sh

@@ -0,0 +1,36 @@
+#!/bin/bash
+# Single partition test - produce and consume from ONE topic, ONE partition
+
+set -e
+
+echo "================================================================"
+echo "    Single Partition Test - Isolate Missing Messages"
+echo "  - Topic: single-test-topic (1 partition only)"
+echo "  - Duration: 2 minutes"
+echo "  - Producer: 1 (50 msgs/sec)"
+echo "  - Consumer: 1 (reading from partition 0 only)"
+echo "================================================================"
+
+# Clean up
+make clean
+make start
+
+# Run test with single topic, single partition
+TEST_MODE=comprehensive \
+TEST_DURATION=2m \
+PRODUCER_COUNT=1 \
+CONSUMER_COUNT=1 \
+MESSAGE_RATE=50 \
+MESSAGE_SIZE=512 \
+TOPIC_COUNT=1 \
+PARTITIONS_PER_TOPIC=1 \
+VALUE_TYPE=avro \
+docker compose --profile loadtest up --abort-on-container-exit kafka-client-loadtest
+
+echo ""
+echo "================================================================"
+echo "                Single Partition Test Complete!"  
+echo "================================================================"
+echo ""
+echo "Analyzing results..."
+cd test-results && python3 analyze_missing.py