test(dsv2lite): add retained HTTP soak gate

[CAICAIIs]

Summary

Adds a retained DeepSeek-V2-Lite HTTP soak runner for issue #465.

The new scripts/bench_dsv2lite_http_soak.py runner reuses the shared /v1/completions leaf benchmark, keeps per-bucket evidence, and writes backend-level plus combined host-staged/NCCL reports. The gate now fails on missing bucket coverage, failed leaf commands, request failures/timeouts, missing required trace coverage, failed clean follow-up, commit/provenance drift, or missing/generic backend runtime boundaries.

Closes #465.

Retained Evidence

Latest retained run:

• commit: a5703d0424d917ce99b4bd8691b0b86eecde966f
• model revision: 604d5664dddd88a0433dbae533b7fe9472482de0
• hardware: 2x RTX 5090, CUDA 12.8
• workload: prompt_words=64, max_tokens=64, greedy, ignore-EOS, concurrency 4,8, duration_s=120, bucket_s=60, num_requests=8
• required trace coverage: 1.0

Results:

Backend	Runtime boundary	Completed	Failed/timeouts	Buckets / leafs	Gate	Artifact SHA-256
host-staged	host-staged	112	0 / 0	4 / 14	passed	3fe4f163024602a51f10cac0c15cc24a5feffd92bbcb93091e175cd63e49bd33
NCCL	NCCL 2.26.2, NCCL_IB_DISABLE=1, NCCL_P2P_DISABLE=1	128	0 / 0	4 / 16	passed	52c2a70d895fc0c080233961ea06824097fbf5b7241a0983e41c0ca6646e762f
combined	host-staged + NCCL	-	-	-	passed	1c06e8825da70888277f1485f54d7f4fb9b2f61d617149d3ac7357cd5a03e7f1

The combined report passed child gates, commit consistency, model provenance consistency, and runtime-boundary retention.

Claim Boundary

This is short-shape sustained HTTP soak evidence for the named backend/runtime contracts. It does not claim production readiness, default-runtime NCCL coverage, long/mixed-prompt soak stability, multi-node recovery, vLLM parity, or a hard latency/memory drift budget.

Validation

python3 -m py_compile scripts/bench_http_common.py scripts/bench_http_serving.py scripts/bench_http_sweep.py scripts/bench_dsv2lite_http_slo.py scripts/bench_dsv2lite_http_soak.py
python3 -m unittest -v tests/test_bench_http_common.py tests/test_bench_http_serving.py tests/test_bench_http_sweep.py tests/test_bench_dsv2lite_http_slo.py tests/test_bench_dsv2lite_http_soak.py
cargo fmt --all --check
cargo metadata --locked --no-deps --format-version 1
git diff --check

python3 -m unittest passed 77 tests.

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