Implement pre-warmed VM pool for fcctl-core

[AlexMikhalev]

Summary

Implement pre-warmed VM pool for sub-500ms allocation as required by terraphim_rlm.

Context

PR #426 (feat(rlm): RLM orchestration with MCP tools) requires pre-warmed VM pool functionality for fast VM allocation.

Requirements

The implementation should provide:

1. Pool management with configurable min/max/target sizes
2. Prewarming manager to maintain pool levels
3. Sub-2 second boot optimization
4. VM allocation from pre-warmed pool in <500ms

Existing Implementation

Some prewarming logic exists in terraphim_firecracker/src/performance/prewarming.rs but needs integration with fcctl-core.

Related

Blocked by: #664 (Create fcctl-core crate)