[AgentX]: Use burst phase starts by default / [AgentX]：默认使用阶段起始突发模式 (#2375)

* feat(agentx): add opt-in burst phase starts

Allow saturation recipes to request AIPerf burst phase starts without changing the default faithful replay behavior. Validate the environment value and cover enabled, disabled, and invalid command generation.

中文：允许饱和度测试配置按需启用 AIPerf 阶段起始突发，同时保持默认的忠实回放行为不变。新增环境变量校验，并覆盖启用、禁用和非法值的命令生成测试。

* refactor(agentx): default to burst phase starts

Start selected AgentX trajectories together by default so configured concurrency is active within the benchmark window. Preserve faithful recorded phase-start spreading as an explicit AIPERF_BURST_PHASE_STARTS=0 override.

中文：默认同步启动选中的 AgentX 轨迹，使配置的并发度能在基准测试窗口内生效；同时保留 AIPERF_BURST_PHASE_STARTS=0，用于显式恢复按记录时间分散启动。

* refactor(agentx): move phase-start default into AIPerf

Update the AIPerf submodule to make burst phase starts the default and remove duplicate InferenceX wrapper plumbing and tests.

中文：更新 AIPerf 子模块，将阶段起始突发设为默认行为，并移除 InferenceX 中重复的封装逻辑和测试。

* chore(agentx): pin merged AIPerf AgentX v1.0 commit

Point the AIPerf submodule at the cquil11/aiperf-agentx-v1.0 merge commit containing the phase-start default change.

中文：将 AIPerf 子模块更新到 cquil11/aiperf-agentx-v1.0 分支中包含阶段起始默认行为变更的合并提交。

Author: cquil11