docs(VT-REFTIER-HOST-ADDRESSABLE): VT_ADOPT_DEVICE_BYTES reaches Metal and integrated ROCm, and its numbers do not (#1502) - #1615

…l and integrated ROCm, and its numbers do not (#1502)

`docs/ENVIRONMENT.md` described the lever's reach as "Vulkan today" and closed
with "No effect on CUDA/CPU/Metal, whose backends do not advertise the property".
Both halves stopped being true at `cffe59b02` (#1477).

That change moved `ReferenceTierEligible` off the wider `UnifiedMemory()` onto
`Backend::DeviceMemoryIsHostAddressable()`, and added truthful overrides so no
backend silently lost the reference tier. `MetalBackend` now answers
`MetalContext::unified_memory()` and `RocmBackend` answers its `unified_memory_`.
The weight loader gates this lever on exactly that predicate, at both
`AdoptDeviceBytesAsHost` branches in `qwen3_5_weights.cpp`, so the lever acts on
Apple silicon and on an integrated ROCm part.

The correction is not "add two backend names". Every number in that row is GB10
through Vulkan, and nobody has measured the lever on either new arm. The row read
as if the measurement covered the reach, so it now says which backends it is
MEASURED on and which merely satisfy the predicate. CUDA and CPU are unchanged
and still inert: neither overrides the default `false`, which
`tests/vllm/platforms/test_platform.cpp` pins for GB10, and the CPU backend
reporting `UnifiedMemory() == true` while the narrower predicate stays `false` is
the whole reason the two properties are separate.

Verified against the tree rather than against the issue: the three overrides, the
two gate sites, the absent CUDA and CPU overrides, and the GB10 assertion were
each read at this head.

The measurement on Metal and on integrated ROCm stays owed. It needs an
Apple-silicon box or an integrated AMD part, which this row has not taken, so it
is recorded under `## Owed` in the spec that widened the predicate rather than
left to be discovered from a document that now promises less than it did.

No checker relates a backend predicate to a prose sentence, and none can be built
cheaply, which is why this landed as a document defect rather than a red gate.

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