Stamp every qa push as a release candidate

Every push to qa now tags v<version>-rc<N>, N incrementing, before the gates
run. That answers "which build is on qa, and is it the one I tested?" without
anyone having to remember it.

The tag lands whether the run goes green or red, deliberately. A red candidate
needs a name more than a green one does: "rc3 failed release-tests on windows"
is a sentence you can act on, "qa is red" is not. Red on qa is the gate doing
its job — the branch saying this is not production worth yet.

release.yml now excludes v*-rc*. Its trigger was v*, which matches the
candidate tags, so without this every push to qa would have built and PUBLISHED
a GitHub release — including for the candidates that failed.

.github/workflows/release.yml

@@ -4,6 +4,11 @@ on:
   push:
     tags:
       - 'v*'
+      # NOT the release-candidate tags. Every push to `qa` stamps a
+      # v<version>-rc<N> so a run can be named, and 'v*' matches those too —
+      # which would have this workflow build and PUBLISH a GitHub release for
+      # every candidate, including the red ones.
+      - '!v*-rc*'
 
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