* fix: recognize working WSL1 without a WSL2 kernel
* fix: recognize unsigned Windows missing-kernel status
* fix(wsl): fold the missing-kernel guest probe into wsl-availability
The separate wsl-missing-kernel-probe module failed three CI gates: it was
not in the web typecheck project (TS6307), it added a new direct wsl.exe
spawn outside wsl-runner, and its `catch { return false }` tripped the
probe-failure-semantics ratchet.
wsl-availability.ts already owns the answer and is already on the invocation
allowlist, so the probe lives there now. A guest probe that cannot spawn
keeps the real --status failure instead of minting a fresh negative, which
is what the ratchet exists to prevent -- and is the more correct semantics.
