feat(builder): append YYYYMMDD to test versions

skills/builder/scripts/version.sh

@@ -11,8 +11,11 @@ With --docker, print the Docker tag rendering of that version.
 
 Derivation (builder contract «版本号»):
   official  HEAD exact-match of vX.Y.Z          → X.Y.Z
-  test      nearest ancestor stable tag         → X.Y.Z~branch.n+gSHA
-  no tag    no stable tag reachable from HEAD   → 0.0.0~branch+gSHA
+  test      nearest ancestor stable tag         → X.Y.Z~YYYYMMDD.branch.n+gSHA
+  no tag    no stable tag reachable from HEAD   → 0.0.0~YYYYMMDD.branch+gSHA
+
+YYYYMMDD is the build date. SOURCE_DATE_EPOCH, if set, is that instant in
+UTC; otherwise the local calendar date. Official X.Y.Z never gets a date.
 
 Stable tags match v<major>.<minor>.<patch> only. Baseline is ancestry, not
 the highest version in the repository. This script does not fetch tags.
@@ -100,6 +103,16 @@ short_sha() {
   gitc rev-parse HEAD | cut -c1-7
 }
 
+# Build-day YYYYMMDD. SOURCE_DATE_EPOCH (UTC) wins when set, for reproducible
+# test packages; otherwise the machine's current local date.
+build_date() {
+  if [[ -n "${SOURCE_DATE_EPOCH:-}" ]]; then
+    date -u -d "@${SOURCE_DATE_EPOCH}" +%Y%m%d
+  else
+    date +%Y%m%d
+  fi
+}
+
 # Highest stable tag pointing at HEAD, or empty.
 official_tag_at_head() {
   local tag best=
@@ -135,7 +148,7 @@ nearest_stable_tag() {
 }
 
 derive_canonical() {
-  local tag base n sha branch
+  local tag base n sha branch ymd
   tag=$(official_tag_at_head)
   if [[ -n "$tag" ]]; then
     printf '%s' "${tag#v}"
@@ -143,13 +156,14 @@ derive_canonical() {
   fi
   sha=$(short_sha)
   branch=$(branch_slug)
+  ymd=$(build_date)
   base=$(nearest_stable_tag)
   if [[ -z "$base" ]]; then
-    printf '0.0.0~%s+g%s' "$branch" "$sha"
+    printf '0.0.0~%s.%s+g%s' "$ymd" "$branch" "$sha"
     return
   fi
   n=$(gitc rev-list --count "${base}..HEAD")
-  printf '%s~%s.%s+g%s' "${base#v}" "$branch" "$n" "$sha"
+  printf '%s~%s.%s.%s+g%s' "${base#v}" "$ymd" "$branch" "$n" "$sha"
 }
 
 to_docker() {