feat(gate): check.sh --brief prints a red lane's first error lines only

frontend/src/lib/gates.snapshot.json

@@ -1436,7 +1436,8 @@
         "e2e/helpers.ts",
         "e2e/nav.spec.ts",
         "e2e/player.spec.ts",
-        "e2e/settings.spec.ts"
+        "e2e/settings.spec.ts",
+        "e2e/speed.measure.ts"
       ],
       "js configs": ["eslint.config.js", "postcss.config.js"],
       "lib tests": [
@@ -2309,16 +2310,40 @@
     "scripts/check.sh": [
       "set -u",
       "cd \"$(dirname \"$0\")/..\" || exit 2",
-      "mode=\"${1:-all}\"",
-      "case \"$mode\" in all | lint) ;; *) echo \"usage: scripts/check.sh [lint]\" >&2; exit 2 ;; esac",
+      "mode=all",
+      "brief=0",
+      "for arg in \"$@\"; do",
+      "  case \"$arg\" in",
+      "    all | lint) mode=\"$arg\" ;;",
+      "    --brief) brief=1 ;;",
+      "  esac",
+      "done",
       "export MSYS_NO_PATHCONV=1 # Git Bash: do not rewrite the in-container paths below",
       "TESTCOMPOSE=backend/docker-compose.test.yml",
       "failed=()",
       "lane() { # lane <name> <command…> — run it, remember a failure, never stop",
       "  local name=\"$1\"",
       "  shift",
-      "  echo \">> $name…\"",
-      "  \"$@\" || failed+=(\"$name\")",
+      "  if [ \"$brief\" = 0 ]; then",
+      "    echo \">> $name…\"",
+      "    \"$@\" || failed+=(\"$name\")",
+      "    return 0",
+      "  fi",
+      "  local out",
+      "  out=\"$(mktemp)\"",
+      "  if \"$@\" >\"$out\" 2>&1; then",
+      "    echo \">> $name: ok\"",
+      "  else",
+      "    failed+=(\"$name\")",
+      "    echo \">> $name: FAILED\"",
+      "    brief_excerpt \"$out\"",
+      "  fi",
+      "  rm -f \"$out\"",
+      "}",
+      "brief_excerpt() {",
+      "  local hits",
+      "  hits=\"$(grep -E -i -A 2 -m 20 'error|fail|✗|×|warn|would reformat|unused|vulnerab|found [0-9]' \"$1\")\"",
+      "  if [ -n \"$hits\" ]; then printf '%s\\n' \"$hits\"; else tail -n 20 \"$1\"; fi",
       "}",
       "ruff_pinned() {",
       "  local pin host",