Make the deploy scripts enforce which branch reaches which server

The rule for this site, stated plainly: only `wj` reaches /shopdb (WJ
production), only `dev` reaches /ops, and `main` is the curated release other
sites adopt and deploys nowhere. The tooling did not know that, and in two
places actively assumed the opposite.

export-github.sh refused --dist on any branch but main, because production was
assumed to install main. It does not - it installs wj. That guard fired today: a
wj export produced its bundle and then died at the dist step, so the dists on
the share came from the main export that happened to run next. The code going to
production and the frontend going to production would have come from different
branches, silently, and nothing downstream could tell.

Now one branch builds one dist: wj builds the /shopdb dist, dev builds the /ops
dist, and any other branch is refused because a dist it built could only ever be
installed by mistake. Each dist carries a .dist-source stamp naming the branch,
commit, base path and build time.

The two deploy scripts copy from the clone on WHATEVER branch it is sitting on
and only ever checked the destination (the web.config mount path), so a clone
left on the wrong branch deployed the wrong code to a correctly-identified
server without a word. Both now refuse unless the clone is on their branch, and
both read the dist's .dist-source and refuse a frontend built from anywhere
else. A directory of hashed filenames says nothing about where it came from, and
robocopy /MIR will mirror whichever one it is pointed at.

pull-shopdb-bundle.cmd and update-shopdb-github.cmd silently defaulted to main.
That was safe when main was the only thing anyone deployed; with three branches
it is a way to carry the wrong one to a server without ever typing its name. The
argument is required now, and the error lists what each branch is for.

tools/mirror-wilm.sh is the missing half of fetch-wilm-branch.cmd: that script
only puts a bundle on the share, and wilm does not appear on gitea by itself.
The new one fetches the carried bundle and pushes wilm to gitea, printing the
bundle's age first - a mirror that looks authoritative and is a week old is
worse than no mirror - and noting that wilm sits on the publication lineage, so
it shares no ancestor with main/wj/dev and cannot be merged here.

Scope: product

tools/export-github.sh

@@ -265,42 +265,61 @@ git bundle create "$BUNDLE" HEAD "$BRANCH" --tags
 git bundle verify "$BUNDLE" >/dev/null
 echo "bundle: $BUNDLE"
 
-# --- 6. optional per-instance dists (both bases: /ops = dev, /shopdb = prod) ---
-# Two instances run on the box; each needs its own base-path build. Always
-# rebuild BOTH so prod never ships a stale frontend.
-# --dist IS MAIN ONLY.
+# --- 6. the frontend dist for THIS branch's instance ---------------------------
 #
-# These paths are not branch-qualified and cannot safely be: update-prod-server
-# .cmd robocopies frontend-dist-subpath-shopdb onto the production server with
-# /MIR, a full mirror, and has no idea which branch built it. So --dist from a
-# site branch would replace production's frontend with unreleased work at the
-# next deploy, silently, and the deploy script could not tell.
+# ONE BRANCH, ONE INSTANCE, ONE DIST. The rule for this site:
 #
-# Branch-qualifying the directory would leave the same trap for whoever pointed
-# a deploy at the wrong one. Production runs the product; a site branch has no
-# business producing the artefact production installs.
-if [ "$BUILD_DIST" = 1 ] && [ "$BRANCH" != "main" ]; then
+#     wj   -> /shopdb   (WJ PRODUCTION)   dist: frontend-dist-subpath-shopdb
+#     dev  -> /ops      (dev instance)    dist: frontend-dist-subpath-ops
+#     main -> nothing   (curated release other sites adopt)
+#
+# This used to build BOTH dists and refuse --dist unless you were on main,
+# because production was assumed to install main. It does not: production
+# installs wj. Under the old rule a wj export failed at this step and the
+# dists on the share came from whichever main export happened to run next -
+# so the code going to prod and the frontend going to prod came from different
+# branches, silently, and the deploy scripts could not tell.
+#
+# Each dist now carries a .dist-source marker naming the branch, the commit and
+# the time that built it. update-prod-server.cmd and update-dev-server.cmd read
+# it and refuse a dist built from the wrong branch, so the trap the old comment
+# worried about is closed by a check rather than by a convention.
+case "$BRANCH" in
+  wj)   DIST_BASE=shopdb ;;
+  dev)  DIST_BASE=ops ;;
+  *)    DIST_BASE='' ;;
+esac
+
+if [ "$BUILD_DIST" = 1 ] && [ -z "$DIST_BASE" ]; then
   echo "REFUSED: --dist on branch '$BRANCH'." >&2
   echo >&2
-  echo "The dist directories are what update-prod-server.cmd mirrors onto the" >&2
-  echo "production server, and they are not branch-qualified. Building them" >&2
-  echo "from '$BRANCH' would put unreleased work on production at the next" >&2
-  echo "deploy, with nothing to reveal it." >&2
+  echo "Only a branch that a server actually runs builds a dist:" >&2
+  echo "    wj  -> /shopdb (production)" >&2
+  echo "    dev -> /ops" >&2
   echo >&2
-  echo "Promote the commits to main and export from there:" >&2
-  echo "    tools/wj-status.sh" >&2
-  echo "    tools/promote-to-main.sh <sha> ..." >&2
+  echo "'$BRANCH' deploys nowhere, so a dist built from it could only ever be" >&2
+  echo "installed by mistake. Export it without --dist." >&2
   exit 1
 fi
 
 if [ "$BUILD_DIST" = 1 ]; then
   cd "$WORK/frontend"
-  for base in ops shopdb; do
-    VITE_BASE_PATH=/$base/ npm run build --silent
-    rm -rf "$OUT/frontend-dist-subpath-$base"
-    cp -r dist "$OUT/frontend-dist-subpath-$base"
-    echo "dist:   $OUT/frontend-dist-subpath-$base/  (base /$base/)"
-  done
+  VITE_BASE_PATH=/$DIST_BASE/ npm run build --silent
+  rm -rf "$OUT/frontend-dist-subpath-$DIST_BASE"
+  cp -r dist "$OUT/frontend-dist-subpath-$DIST_BASE"
+
+  # Provenance, read by the deploy scripts. A dist is a directory of hashed
+  # filenames with nothing in it that says where it came from, and robocopy
+  # /MIR will happily mirror the wrong one onto a correctly-identified server.
+  {
+    echo "branch=$BRANCH"
+    echo "commit=$(git -C "$WORK" rev-parse HEAD)"
+    echo "base=/$DIST_BASE/"
+    echo "built=$(date '+%Y-%m-%d %H:%M:%S %z')"
+    echo "host=$(hostname)"
+  } > "$OUT/frontend-dist-subpath-$DIST_BASE/.dist-source"
+
+  echo "dist:   $OUT/frontend-dist-subpath-$DIST_BASE/  (base /$DIST_BASE/, from $BRANCH)"
 fi
 
 cat <<EOF