cold-start controls (warm pools, min replicas, idle policies)

## Summary
Add explicit cold-start controls and warm-pool strategies.

## Motivation
Latency-sensitive workloads need predictable startup behavior and configurable cost/performance tradeoffs.

## Proposal
- Min warm instances per language/project/function.
- Keep-warm pools and prefetch hooks for hot paths.
- Idle shutdown policy tiers.
- Cold-start telemetry and SLO reporting.

## Acceptance criteria
- Operators can configure warm capacity and idle policy.
- Cold-start latency is measurable and visible.
- Keep-warm behavior integrates with deprovision logic.

No files, tests, or entry points are named. Start by locating the configuration and deprovision logic, then trace how function capacity is managed. Done means operators can configure warm capacity and idle policies, cold-start latency is visible, and keep-warm behavior integrates with deprovisioning.

Written by the indexing model from the issue text.