Pre-load CAG cache

Explicitly warm up the CAG cache for a POT.

This loads:

• All Constitution facts (Layer 1)
• Facts with POT Score >= threshold (Layer 2)

Useful for:

• Pre-loading cache before expected queries
• Ensuring cache is ready for low-latency responses
• Verifying cache initialization works correctly

{
  "cache_stats": {
    "certainty_size": 47,
    "constitution_size": 5,
    "initialized_at": "2026-01-20T10:30:00Z",
    "total_size": 52
  },
  "status": "warmed"
}