fix(legacy): align RubricGroup.score_group metrics and advantages with score_rollout

[crazywriter1]

Supersedes #2009 — same fix, rebased onto current main after #2303 moved v0 rubrics to verifiers/legacy/.

Summary

• RubricGroup.score_group summed child rewards correctly but left per-child advantage on state and often left stale trajectory advantage/reward from earlier children.
• Metrics were merged into preexisting keys, unlike score_rollout and Rubric.score_group, which fully replace state["metrics"].
• After aggregation: recompute advantage from summed rewards, fill trajectory fields only when still None, and replace metrics entirely.
• Restore advantage and trajectory fields between child rubrics so partial writes do not leak.

Risk / notes

• Aggregate rewards were already correct; this is contract/consistency cleanup for legacy group scoring.
• Default Rubric.has_advantages is still False; trainers that recompute advantages from rewards are largely unaffected.

Test plan

• Manual: children [[1,0],[0,1]] → rewards [1,1], advantages [0,0]
• Manual: preset trajectory step reward preserved; unset steps still get aggregate fill
• Manual: preexisting stale metrics cleared after group scoring
• Manual: child with empty metrics → state["metrics"] == {}

Note

Fix RubricGroup.score_group to align metrics and advantages with score_rollout

• Adds an early return with a warning when score_group receives zero states.
• Snapshots each state's reward, metrics, advantage, and per-step trajectory values before invoking child rubrics, then restores them after each child so all rubrics see identical inputs.
• Accumulates rewards and metrics across child rubrics, then writes the final aggregated reward to each state and sets state['advantage'] as aggregated_reward - average_reward.
• Per-step trajectory reward and advantage are only written when currently None, matching score_rollout behavior.
• Behavioral Change: metrics aggregation now always writes a fresh dict per state rather than conditionally merging into an existing one.

^{Macroscope summarized b07b7f1.}