feat: Carrier thread analysis and utilization metrics

[rlaope]

Problem

Virtual threads run on carrier (platform) threads, but Argus currently doesn't provide visibility into:

• Which carrier threads are being used
• Carrier thread utilization rates
• How virtual threads are distributed across carriers
• Potential carrier thread bottlenecks

Proposed Solution

Add carrier thread analysis features:

1. Track carrier thread assignments from JFR events
2. Calculate utilization per carrier thread
3. Show distribution of virtual threads across carriers
4. Alert when carrier pool is saturated

Acceptance Criteria

• Extract carrier thread info from JFR events
• Track active virtual threads per carrier
• Calculate carrier utilization (busy time / total time)
• Add /carrier-threads API endpoint
• Add carrier analysis section to dashboard
• Show carrier thread count vs virtual thread count ratio
• Visualize carrier utilization as a heatmap

Technical Considerations

• Carrier thread ID available in jdk.VirtualThreadStart event
• Need to correlate START and END events to calculate duration
• Consider ForkJoinPool default carrier pool behavior
• Track context switches between virtual threads on same carrier

API Design

GET /carrier-threads

{
  "carrierThreadCount": 8,
  "virtualThreadCount": 1523,
  "ratio": 190.4,
  "carriers": [
    {
      "carrierId": 1,
      "name": "ForkJoinPool-1-worker-1",
      "currentVirtualThreads": 3,
      "totalVirtualThreadsHandled": 245,
      "utilizationPercent": 78.5,
      "avgVirtualThreadDuration": 12.5
    }
  ]
}

UI Mockup

┌─────────────────────────────────────────────────────────────┐
│ Carrier Thread Analysis                                     │
├─────────────────────────────────────────────────────────────┤
│ Carrier Threads: 8    Virtual Threads: 1,523    Ratio: 190x │
├─────────────────────────────────────────────────────────────┤
│ Carrier Utilization Heatmap                                 │
│                                                             │
│ FJP-1-worker-1  ████████████████████░░░░░  78%             │
│ FJP-1-worker-2  ██████████████████████████  92%             │
│ FJP-1-worker-3  ████████████░░░░░░░░░░░░░  45%             │
│ FJP-1-worker-4  ██████████████████████░░░  85%             │
│ ...                                                         │
└─────────────────────────────────────────────────────────────┘

Metrics to Track

• Virtual threads per carrier (current, max, avg)
• Carrier busy time percentage
• Context switch rate
• Queue depth per carrier (if accessible)