Cache a negative POSIX capability probe

[OskarEichler]

Summary

When Windows cannot fork, each OS.posix? call repeats the failing capability probe because ||= never caches false. Return an already-computed false value, while still permitting a nil reset.

Reproduction and verification

Ten stubbed Windows calls return false with one NotImplementedError probe rather than ten. Non-Windows returns true without forking. No subprocess was created by the capability repro.

• External focused repro corpus: 12 checks pass against this isolated branch, under rbenv Ruby 4.0.6.
• Existing upstream suite: 36 examples, zero failures before and after the patch (RBENV_VERSION=4.0.6 rbenv exec rspec -Ilib -Ispec).
• Source-only change; no existing tests modified or new test files added, per the contributor's task constraint. The focused corpus lives outside the repository.
• Local verification was on macOS. Other Ruby versions and native operating systems were not executed. Upstream RuboCop configuration cannot load its legacy rubocop-lts1_8.yml, including after installing current rubocop-lts; lint status is not represented as passing.

Breaking-change notes

No API removal or new platform assumption. Like the existing true cache, a negative result is retained until explicitly reset to nil.