feat: report evaluation wall time

[skirdey]

Summary

• measure end-to-end evaluation wall time in RolloutManager.eval
• report it as perf/eval_time through the existing eval metrics path
• preserve custom evaluator metrics without mutating their dictionary

Motivation

Rollout generation already reports perf/rollout_time, but periodic evaluation has no corresponding duration. On long-running jobs this makes evaluation pauses indistinguishable from rollout, training, or weight-sync stalls.

Test plan

• added a unit test for custom metric preservation and wall-time reporting
• python3 -m py_compile slime/ray/rollout.py tests/test_rollout_metrics.py
• isolated _eval_metrics contract execution passed
• git diff --check

The full tests/test_rollout_metrics.py requires the project CUDA/SGLang test environment and is left to upstream CI.