fix(session diff): diff lines miscalculation

[nyepppp]

Description

情况1：越跑越少；
情况2：不增加了；

Steps to Reproduce

Expected Behavior

diff和session会话增加行数对齐

Actual Behavior

1.不一致，有时候会出现会减少。
2.当前同步机制貌似是跟着轮次变动更新

Environment

• OS: windows 11

Logs / Screenshots

Additional Context

[Harry19081]

@Neonforge98 and @VantaNode , any of you want to claim this issue?

[beruro]

Taking this one. Will investigate the diff line miscalculation and the per-round sync mechanism.

[Harry19081]

@beruro this is already fixed.

[beruro]

修复说明 / Fix Summary

根因 / Root cause: persist_runtime_edit_artifacts 在每个 runtime batch 处理时会用当前 batch 的 diff 覆盖 session 的 final diff，而不是累加整个 session 的所有 chunk。因此当一次编辑跨多个 runtime batch 完成时，final diff 的行数统计只反映最后一个 batch，导致 session diff 行数计算错误。

The runtime was overwriting the session's final diff on each batch instead of accumulating full-session chunks, so multi-batch edits reported diff line counts from only the last batch.

修复 / Fix: 新增 recompute_session_final_diffs，在持久化 runtime 编辑产物后，基于该 session 所有受影响文件（touched files）重新累加计算 final diff，保证跨 batch 的 added/removed 行数正确聚合。补充了 runtime_artifact_tests 覆盖：跨 batch 累加、无 touched files 时的 no-op、以及多文件独立统计。

Added recompute_session_final_diffs to recompute the final diff by accumulating across all touched files after persisting runtime artifacts, plus tests covering cross-batch accumulation, empty no-op, and independent multi-file tracking.

Commit: 44c22d6 (fix(session): accumulate final diff across runtime batches (#192))