fix(ci): start release notes from a tag that exists

The notes range was built from the previous release's *name*, never from a check
that it exists. 1.9.3 shipped only as rc.1, so v1.9.4-rc.1 asked git for
v1.9.3..v1.9.4-rc.1 and the publish step died on `unknown revision` -- after all
four platforms had already built and uploaded their artifacts.

The existing rc walk-down covers a skipped RC. Nothing covered a skipped stable,
and that loop does not run at all for an rc.1 (`for n = 0; n >= 1`).

This is not only about release notes: publish-msstore has `needs:
publish-release`, so the same missing tag would have taken the Store deployment
down with it on a stable promotion, which is the one place that failure is
expensive to discover.

Falls back to the nearest tag reachable from the release commit's parent, which
is what "since the last release" meant to begin with -- v1.9.2 for the tag that
failed, a 25-commit range. If no tag is reachable at all, the root commit keeps
the range valid rather than handing git an empty left-hand side.

Author: EtienneLescot

.github/workflows/build.yml

@@ -745,6 +745,24 @@ jobs:
               fi
             done
           fi
+          # Everything above computes what the previous release was *called* and
+          # never checks that it exists. A version line that stopped at its RC
+          # makes that a name for nothing: 1.9.3 shipped only as rc.1, so
+          # v1.9.4-rc.1 asked git for v1.9.3..v1.9.4-rc.1 and the publish step
+          # died on `unknown revision` -- after all four platforms had already
+          # built, and with publish-msstore sitting behind publish-release, so
+          # the same gap would silently block a stable release's Store
+          # deployment too. The rc walk-down above handles a skipped RC; nothing
+          # handled a skipped stable, and it does not run at all for an rc.1.
+          if ! git rev-parse -q --verify "refs/tags/${NOTES_START_TAG}" >/dev/null; then
+            # The nearest tag reachable from the release commit's parent, which
+            # is what "since the last release" meant in the first place. Empty
+            # only when no tag is reachable at all (a first release), so fall
+            # back to the root commit to keep the range valid either way.
+            FALLBACK="$(git describe --tags --abbrev=0 "${TAG}^" 2>/dev/null || true)"
+            NOTES_START_TAG="${FALLBACK:-$(git rev-list --max-parents=0 HEAD | tail -1)}"
+            echo "Previous-release tag did not exist; starting notes from ${NOTES_START_TAG}"
+          fi
           echo "Computed notes_start_tag=${NOTES_START_TAG} for tag=${TAG}"
 
           echo "tag=$TAG" >> "$GITHUB_OUTPUT"