[experimental chore/agentx-v0.2-aiperf-testing branch] agentic build_replay_cmd: --cache-bust system_prefix mismatched with current aiperf scenario (requires first_turn_prefix)

## Summary `benchmarks/benchmark_lib.sh` (function `build_replay_cmd`, around line 1000 of v0.2 tip) passes `--cache-bust system_prefix` to `aiperf profile`, but the current `utils/aiperf` submodule (`70fecb2e`) hardcodes `first_turn_prefix` as the only valid value for the `inferencex-agentx-mvp` scenario. The validation is fatal: ``` Value error, Scenario invariants violated (1 conflict): - --cache-bust: got 'system_prefix', required 'first_turn_prefix' (scenario 'inferencex-agentx-mvp' requires cache_bust.target=first_turn_prefix; got system_prefix) Pass --unsafe-override to convert to warnings (run will be marked submission_valid=false). ``` This is masked for short smoke runs because `build_replay_cmd` auto-adds `--unsafe-override` when `DURATION < 900`. Production sweep configs use `DURATION=1800` and hit the error directly — the run flips to "invalid" or fails at validation, depending on flags. ## Repro ```bash # Any cpu/noffload sweep with DURATION=1800 (the production default) podman run ... \ -e MODEL=MiniMaxAI/MiniMax-M2.5 -e TP=4 -e CONC=16 \ -e OFFLOADING=cpu -e TOTAL_CPU_DRAM_GB=1200 \ -e DURATION=1800 \ ... /workspace/benchmarks/single_node/agentic/minimaxm2.5_fp8_mi355x.sh # vllm starts cleanly (Application startup complete, /health 200), then # aiperf profile bails immediately with the validator error above. ``` ## Suggested fix ```diff - REPLAY_CMD+=" --cache-bust system_prefix" + REPLAY_CMD+=" --cache-bust first_turn_prefix" ``` …or, if backwards compat with older aiperf submodules matters, source the value from the scenario plugin via `aiperf scenario describe inferencex-agentx-mvp` and inject it dynamically. After this patch, the full 30-min sweep runs cleanly (verified with both `OFFLOADING=none` and `OFFLOADING=cpu`, 11 + 8 conc points respectively, 0 launcher errors). The launcher comment block at line 950 should also be updated — it currently says: ``` # The scenario plugin locks: --cache-bust system_prefix, ``` which became stale when the scenario validator was tightened. ## Environment - Branch: `chore/agentx-v0.2-aiperf-testing` (tip `c8dfb585`) - aiperf submodule: `70fecb2e refactor(input): move --max-context-length filter into WekaTraceLoader`