docs: the blind spot CI found is hardware, not platform

This file already warns that a local Windows run says nothing about Linux or
macOS. The two entities CI rejected were not that: macOS passed, and the two
failing jobs were on different operating systems. What those runners share is
that they are virtual machines and this desktop is not.

That axis is invisible to the cross-target checks this file recommends as the
cheap substitute for other platforms -- the code compiles identically and only
the readings differ, so both `cargo check --target` commands would have passed.
Anything resolving off SMBIOS, a device enumeration or a bus classification can
differ this way.

Also records a loose thread: memory.bandwidth.{achievable,stream_triad} declare
`derived_from` including `generation`, and both resolved on CI while
`generation` was unavailable.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>