test(VT-REFTIER-HOST-ADDRESSABLE): check CUDA's host-addressability answer in the two halves it actually has

[localai-bot]

CudaBackend::DeviceMemoryIsHostAddressable() gates the portable CPU reference
tier (src/vt/op_provider.cpp), the weight loader's VT_ADOPT_DEVICE_BYTES
adoption (src/vllm/model_executor/models/qwen3_5_weights.cpp) and the
logits-processor bounce (src/vllm/v1/sample/logits_processor/builtin.cpp).
Being wrong there hands a device pointer to a host memcpy, which is the SIGSEGV
that #844 and #1435 measured. Nothing in the tree held that answer.

The test cited as the pin reads a different backend:
FakeUnifiedAddressablePlatform in tests/vllm/platforms/test_platform.cpp
reports device_type() == kCUDA while its backend() returns
vt::GetBackend(DeviceType::kCPU), so its CHECK_FALSE measures the CPU
backend. #1639 corrected that fixture's comment; this change supplies the pin
the record was still owed.

A runtime pin was rejected on a CI fact, not a preference

No job in .github/workflows/ci.yml has a GPU. cuda-fat-build is the only job
with a CUDA toolchain, it runs the nvidia/cuda:13.3.0-devel container on
ubuntu-latest, it configures -DVLLM_CPP_BUILD_TESTS=OFF and it builds the
vllm target alone. The CUDA registrar returns early when cudaGetDeviceCount
finds no device, so vt::GetBackend(kCUDA) throws on every machine this
project's CI owns. A TEST_CASE reading the real backend would report a skip on
every lane forever, and a skip reads as a pass. That is the shape of evidence
#1635 was filed about, so writing one and calling the debt discharged would
repeat the defect rather than close it.

One claim, two halves, each checked where it executes

The record's claim decomposes, and neither half alone is sufficient.

CudaBackend's answer IS the base default. src/vt/cuda/cuda_backend.cu
gains a static_assert beside the class requiring
decltype(&CudaBackend::DeviceMemoryIsHostAddressable) to be
bool (Backend::*)() const. Taking the address of an inherited member through a
derived class yields a pointer-to-member of the class that DECLARES it, so that
type holds exactly while CudaBackend declares no override of its own, and
becomes bool (CudaBackend::*)() const the moment somebody adds one. It fires on
ANY override, including one returning false, because an override invalidates
the reasoning whatever it returns. cuda-fat-build compiles this translation
unit on every push.

The base default is false. tests/vt/test_backend.cpp gains a Backend
subclass that implements the pure virtuals and deliberately declares no
DeviceMemoryIsHostAddressable. Every other fake in the tree overrides that
method and takes the answer as a constructor argument, so each measures its own
override and none reads the default. This one does, and it answers
UnifiedMemory() == true on purpose so the case cannot pass by the two
predicates happening to agree. Every host lane runs it.

Together they are a mechanical proof of CudaBackend answering false, checked
on surfaces that execute rather than skip.

Mutation evidence

Half one, against nvcc 13.3.33 from the CI container, compiling the real file:

Tree	nvcc -std=c++20 -Iinclude -Isrc -c src/vt/cuda/cuda_backend.cu
clean, sha256 d9d11f96...	rc=0
+ bool DeviceMemoryIsHostAddressable() const override { return true; }	rc=2, static assertion failed at cuda_backend.cu(363)
same override returning false	rc=2, identical message
restored, sha256 d9d11f96...	rc=0

Half two, include/vt/backend.h default flipped to true and the target
rebuilt:

Tree	test_backend -tc="Backend::DeviceMemoryIsHostAddressable defaults to false"
clean, sha256 80edf388...	assertions: 3 | 3 passed, rc=0
default false -> true	assertions: 3 | 1 passed | 2 failed, rc=1
restored, sha256 80edf388...	assertions: 3 | 3 passed, rc=0

Both mutated builds compiled, so neither red is a build failure wearing a test
failure, and both files were restored byte-for-byte and re-verified by sha256
and by re-running the check.

Suites: test_backend, test_backend_cross_device,
test_backend_cross_device_vt_attn_decode_d128, test_cuda_backend,
test_op_provider, test_reference_tier -- 6/6 pass; test_backend alone is
9 cases / 51 assertions.

What is still not held, stated rather than left to be inferred

No CI surface observes a live CudaBackend object answering the question,
because no CI surface has a device. tests/vt/test_cuda_backend.cpp gains that
observation using the skip convention every case in that file already follows,
and on a CPU host it reports assertions: 0 and says in its own skip message
that nothing was observed and where the answer actually lives. It is the
empirical belt to the two structural braces. Citing it as the pin would repeat
#1635 exactly, and both the case and the spec say so.

docs/ENVIRONMENT.md claimed nothing pinned the real CudaBackend, which this
change makes false, so it now names both halves and warns off the wrong
citation. The #1502 row of .agents/issue-index.md keeps the original wrong
citation, because that index is append-only and can never be edited; no row is
edited here.

Fixes #1635

FOLLOWING_AGENTS_PROTOCOL

Following-Agents-Protocol: true
AI-Assisted: true
Assisted-by: Claude:claude-opus-5 [Claude Code]