ROCm MoeSiluMul bf16 is not bit-exact against the CPU reference on gfx1200 (test_backend_cross_device:2063)

test_backend_cross_device fails one assertion on gfx1200 / ROCm 7.2.3:

tests/vt/test_backend_cross_device.cpp:2063: ERROR: CHECK( got == ref_b ) is NOT correct!
[doctest] test cases:  21 |  20 passed | 1 failed | 0 skipped
[doctest] assertions: 360 | 359 passed | 1 failed |

Line 2063 is the bf16 arm of the kMoeSiluMul case, which asserts exact
equality against the CPU reference on the stated grounds that a single multiply
plus an RNE store is exact on both sides:

vt::MoeSiluMul(q, tout, tg, tu);
std::vector<uint16_t> got(n);
doutb.Download(got.data());
CHECK(got == ref_b);  // single multiply + RNE store both sides: exact

So either the ROCm kMoeSiluMul bf16 path is not doing a single multiply with
an RNE store, or that comment's premise does not hold on this device. The f32
arm of the same case passes.

kMoeSiluMul is registered for ROCm at src/vt/rocm/rocm_ops.hip:182; the
kernel is in src/vt/rocm/rocm_moe_router.hip.

Pre-existing, with a control

Found while gating an unrelated row (#1506, the ROCm keep-quant GEMM). It is
not caused by that work, and the control is empirical rather than
argumentative: removing the new src/vt/rocm/rocm_quant_dot.hip from the build
entirely and rebuilding reproduces the identical failure, same line, same
359/360. The new TU also contains zero references to MoeSiluMul.

Not fixed in that flow because it is not a small clear fix: it is a numeric
divergence in a different kernel family that wants its own investigation and
its own red-first gate. Owned by BACKEND-ROCM.

Repro

nix develop .#rocm-shell --command bash -c \
  'cmake -S . -B build-hip -G Ninja -DVLLM_CPP_HIP=ON \
     -DVLLM_CPP_HIP_ARCHITECTURES=gfx1200 -DROCM_PATH=$ROCM_PATH \
     -DCMAKE_BUILD_TYPE=Release && \
   cmake --build build-hip --target test_backend_cross_device -j 6 && \
   ./build-hip/tests/test_backend_cross_device'

Run the provided gfx1200 ROCm reproduction and inspect the bf16 assertion at tests/vt/test_backend_cross_device.cpp:2063. Trace kMoeSiluMul from its registration in src/vt/rocm/rocm_ops.hip:182 into src/vt/rocm/rocm_moe_router.hip, comparing the ROCm result with the CPU reference and the passing f32 arm. Done means the bf16 check passes without regressing the existing cross-device tests.