chore(gate): check.sh can list its lanes and run one of them

--list prints the lane names, --only "<lane>" runs that one lane, so a runner can schedule the lanes itself and a red lane can be re-run on its own. An --only run reports that lane, never the gate, and an unknown name exits 2 instead of passing with nothing run; the contract test holds both.

frontend/src/lib/gates.snapshot.json

@@ -1299,16 +1299,24 @@
       "cd \"$(dirname \"$0\")/..\" || exit 2",
       "mode=all",
       "brief=0",
-      "for arg in \"$@\"; do",
-      "  case \"$arg\" in",
-      "    all | lint) mode=\"$arg\" ;;",
+      "list=0",
+      "only=\"\"",
+      "usage() { echo 'usage: scripts/check.sh [lint] [--brief] [--list | --only \"<lane>\"]' >&2; exit 2; }",
+      "while [ $# -gt 0 ]; do",
+      "  case \"$1\" in",
+      "    all | lint) mode=\"$1\" ;;",
       "    --brief) brief=1 ;;",
+      "    --list) list=1 ;;",
+      "    --only) [ $# -ge 2 ] || usage; only=\"$2\"; shift ;;",
       "  esac",
+      "  shift",
       "done",
+      "[ \"$list\" = 1 ] && [ -n \"$only\" ] && usage",
       "export MSYS_NO_PATHCONV=1 # Git Bash: do not rewrite the in-container paths below",
       "TESTCOMPOSE=backend/docker-compose.test.yml",
       "failed=()",
       "timings=()",
+      "ran=0",
       "ms() { date +%s%3N; }",
       "secs() { awk -v t=\"$1\" 'BEGIN { printf \"%.1f\", t / 1000 }'; }",
       "took=\"\"",
@@ -1316,7 +1324,7 @@
       "  took=\"$(secs $(($(ms) - $3)))\"",
       "  timings+=(\"$(printf '%8s s  %s' \"$took\" \"$1\")$([ \"$2\" = red ] && echo '  (red)')\")",
       "  if [ -n \"${GATE_TIMES_FILE:-}\" ]; then",
-      "    printf '%s\\t%s\\t%s\\t%s\\t%s\\t%s\\n' \"$(date -u +%FT%TZ)\" \"$mode\" \"$1\" \"$took\" \"$2\" \\",
+      "    printf '%s\\t%s\\t%s\\t%s\\t%s\\t%s\\n' \"$(date -u +%FT%TZ)\" \"${only:+only}${only:-$mode}\" \"$1\" \"$took\" \"$2\" \\",
       "      \"$(git rev-parse --short 'HEAD^{tree}' 2>/dev/null)\" >>\"$GATE_TIMES_FILE\"",
       "  fi",
       "  return 0",
@@ -1324,6 +1332,9 @@
       "lane() { # lane <name> <command…> — run it, remember a failure, never stop",
       "  local name=\"$1\" t0 result=ok",
       "  shift",
+      "  if [ \"$list\" = 1 ]; then echo \"$name\"; return 0; fi",
+      "  [ -n \"$only\" ] && [ \"$name\" != \"$only\" ] && return 0",
+      "  ran=$((ran + 1))",
       "  t0=\"$(ms)\"",
       "  if [ \"$brief\" = 0 ]; then",
       "    echo \">> $name…\"",
@@ -1407,14 +1418,21 @@
       "  lane \"pip-audit (backend lock)\" pip_audit",
       "  lane \"npm audit (production dependencies)\" frontend npm audit --omit=dev --audit-level=high",
       "fi",
+      "[ \"$list\" = 1 ] && exit 0",
+      "if [ -n \"$only\" ] && [ \"$ran\" = 0 ]; then",
+      "  echo \"no lane named '$only' in mode $mode — see: scripts/check.sh $mode --list\" >&2",
+      "  exit 2",
+      "fi",
+      "scope=\"gate ($mode)\"",
+      "[ -n \"$only\" ] && scope=\"lane '$only' only (not the gate)\"",
       "echo \">> lane times:\"",
       "printf '%s\\n' \"${timings[@]}\"",
       "echo \"$(printf '%8s' \"$(secs $(($(ms) - gate_t0)))\") s  total\"",
       "if [ ${#failed[@]} -eq 0 ]; then",
-      "  echo \">> gate ($mode): green ✓\"",
+      "  echo \">> $scope: green ✓\"",
       "  exit 0",
       "fi",
-      "echo \">> gate ($mode): RED — ${failed[*]}\" >&2",
+      "echo \">> $scope: RED — ${failed[*]}\" >&2",
       "exit 1"
     ]
   }