fix(macos): do not report the machine model as the CPU model

read_cpu_stats fell back to `hw.model` when `machdep.cpu.brand_string` was
missing. `hw.model` is the machine identifier — "Mac14,2" — so the fallback
turned an unreadable CPU name into a confident wrong reading rather than an
absence. The comment above it claimed the opposite, saying the value was
"left empty rather than filled with a guess" while the next line filled it
with a guess.

It also made the result unreadable. CI went green with both sysctls in play
and nothing distinguished which had answered, so `cpu.model` on the runner
might have been a processor name or a chassis code.

Now the brand string only, empty when absent, which the resolver reports as
unavailable with a reason.

This may turn `non_nullable_entities_are_never_null` red on macOS, since
`cpu.model` is declared non-nullable. That is the point of pushing it: if
the brand string is not readable there, the honest state is a failing
conformance check and a decision about the entity or another source — not a
chassis code standing in for a CPU.

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