fix(runners): scope b200-dgxc qwen3.5 fp4 model path to the V2 checkpoint / 修复：将 b200-dgxc 的 qwen3.5 fp4 模型路径限定到 V2 检查点

[Ankur-singh]

Targets qwen3.5-fp4-nvfp4-v2 (the branch behind #2205), not main.

Problem

In runners/launch_b200-dgxc.sh at #2205's head, the model-path branch keys only on MODEL_PREFIX + PRECISION:

elif [[ $MODEL_PREFIX == "qwen3.5" && $PRECISION == "fp4" ]]; then
    export MODEL_PATH="/scratch/fsw/models/Qwen3.5-397B-A17B-NVFP4-V2"

But two checkpoints now share that pair:

Config key	model: at head
qwen3.5-fp4-b200-sglang, -sglang-mtp	nvidia/Qwen3.5-397B-A17B-NVFP4-V2
qwen3.5-fp4-b200-trt, -trt-mtp	nvidia/Qwen3.5-397B-A17B-NVFP4

Because the script later does export MODEL="$MODEL_PATH", the two TRT configs would serve V2 weights while publishing results under the old checkpoint name.

It is also runner-dependent: the b200 pool spans b200-cw_*, b200-nb_* and b200-dgxc_*, and only launch_b200-dgxc.sh pins a path. So the same config key served old weights on cw/nb and V2 on dgxc, depending on scheduling.

Origin: 25ef4a1 ("remove trt configs") reverted the TRT keys to NVFP4 but left the launcher on the V2 path — a partial revert.

Fix

Branch on the checkpoint so each key resolves to its own weights. The old-checkpoint arm restores the exact /lustre path used on main.

Verification

Extracted the real resolution chain from the file and ran the configs through it:

Config (model)	main	before (bug)	after (fix)
…NVFP4-V2 (sglang)	/lustre/…NVFP4	/scratch/…NVFP4-V2	/scratch/…NVFP4-V2 ✅
…NVFP4 (trt)	/lustre/…NVFP4	/scratch/…NVFP4-V2 ❌	/lustre/…NVFP4 ✅

bash -n clean; fp8 and bf16 qwen3.5 arms unaffected.

Because the TRT arm now resolves identically to main, TRT behaviour is unchanged and no re-sweep is needed; #2205's existing sweep covers the sglang keys it actually changes.

Draft because #2205 itself still has open items (a CHANGES_REQUESTED review, and the SGLang cookbook still documenting the pre-V2 checkpoint). Merging this into that branch changes its head SHA, so its sweep evidence will need a /reuse-sweep-run authorization afterwards.

中文：launch_b200-dgxc.sh 的 qwen3.5+fp4 分支仅依据 model-prefix 与 precision 判断，而这两个字段现对应两个检查点，导致 TRT 配置在 b200-dgxc 运行器上加载 V2 权重却以旧检查点名称发布结果。改为按检查点分支；旧检查点恢复 main 上的 /lustre 路径，故 TRT 行为不变、无需重跑 sweep。