chore(ci): make the Windows checks an ordinary gate (#396)

The three checks carried `continue-on-error` with a summary step failing the job at the end, so that one run could report `fmt`, `clippy` and `test` together rather than stopping at the first and hiding the rest. That was deliberate and temporary, and #392 said to remove it once #366 and #367 closed. Both are closed, and run 32726116136 on `main` reported all three green with "All native Windows checks passed", so the scaffolding goes.

It earned its keep twice on the way out. Both issues turned out to undercount, for the same reason in each case: nothing could see past the first thing that stopped the build. #366 reported four failing tests and had seven, because the crate had grown since it was filed. #367 reported four clippy sites and had nine, because clearing the lib let clippy reach the bin for the first time. Neither gap would have been visible in a run that halted at `fmt`.

The summary step goes with the flags rather than staying behind. Without `continue-on-error` its failure branch is unreachable, and a step whose error handling can never run is the kind of thing this repository keeps deleting rather than keeping for later. Each check now fails the job at its own step, with cargo's own output, which is a clearer signal than a table of outcomes.

The two diagnostic steps keep `continue-on-error`, because a probe that cannot build is not a reason to fail a run whose checks passed. `all-smi doctor` additionally gains `if: always()`, so it still runs after a failed check. That is when knowing what the host is matters most, and previously a red check skipped it.

Refs #366, #367, #392