Turboquant native fp8 v4 store

[sladyn98]

Related to #40069.

Add a native CUDA store path for TurboQuant turboquant_k8v4 FP8-key / 4-bit-value
KV cache storage on Hopper+ GPUs.

The existing Triton store path remains the fallback. The native path is only
selected when the request matches the supported contract:

• FP8 key path
• 4-bit value quantization
• CUDA device using Hopper+ E4M3 FP8
• contiguous int32 slot_mapping
• native op is built and available

This also adds a kill-switch through VLLM_DISABLE_TURBOQUANT_NATIVE_STORE and
validation guards in the C++ wrapper.

Test Plan

• Add byte-parity coverage comparing the native CUDA store against the existing
  Triton store.
• Cover fp16 and bf16 inputs.
• Cover D=127, D=128, and D=256.
• Cover FP8 saturation with large key values.
• Cover negative slot mappings.
• Add an H100 Buildkite lane for the native store parity test so it does not
  silently skip on non-Hopper CI workers.

Test Result

Local static checks passed:

git diff --check main...HEAD
python3 -m py_compile \
  vllm/v1/attention/ops/triton_turboquant_store.py \
  vllm/_custom_ops.py \
  vllm/envs.py \
  tests/quantization/test_turboquant.py
uvx ruff check \
  vllm/v1/attention/ops/triton_turboquant_store.py \
  vllm/_custom_ops.py \
  vllm/envs.py \
  tests/quantization/test_turboquant.py
uvx ruff format --check \
  vllm/v1/attention/ops/triton_turboquant_store.py \
  vllm/_custom_ops.py \
  vllm/envs.py \
  tests/quantization/test_turboquant.py

I could not run the CUDA parity test locally because this machine does not have a
CUDA/Hopper environment. The PR adds a dedicated H100 CI lane to run:

pytest -v -s quantization/
test_turboquant.py::TestStoreDecodeRoundTrip::test_native_fp8_v4_store_matches_trit
on

———
<details>
<summary> Essential Elements of an Effective PR Description Checklist </summary>

- [x] The purpose of the PR, such as "Fix some issue (link existing issues this PR
  will resolve)".
- [x] The test plan, such as providing test command.
- [x] The test results, such as pasting the results comparison before and after, or
  e2e results.
- [x] Documentation update considered. Not needed because this is an internal
  kernel dispatch optimization with Triton fallback and no user-facing API change.
</details>

BEFORE SUBMITTING, PLEASE READ https://docs.vllm.ai/en/latest/contributing
(https://docs.vllm.ai/en/latest/contributing)