js-on-sx: baseline commit (278/280 unit, 148/148 slice, runner stub)

Initial commit of the lib/js/ tree and plans/ directory. A previous
session left template-string work in progress — 278/280 unit tests pass
(2 failing: tpl part-count off-by-one, escaped-backtick ident lookup).
test262-runner.py and scoreboard are placeholders (0/8 with 7 timeouts);
fixing the runner is the next queue item.

plans/restore-loop.sh

@@ -0,0 +1,65 @@
+#!/usr/bin/env bash
+# restore-loop.sh — print recovery state for the js-on-sx loop.
+#
+# Claude Code can't be driven from shell, so this script shows you where things
+# stand and points at the briefing file. To respawn the agent, feed
+# plans/agent-briefings/loop.md into Claude via the Agent tool with
+# run_in_background=true.
+#
+# Usage:
+#   bash plans/restore-loop.sh          # status snapshot
+#   bash plans/restore-loop.sh --print  # also cat the briefing
+#
+set -uo pipefail
+
+cd "$(dirname "$0")/.."
+
+echo "=== js-on-sx loop state ==="
+echo
+echo "Branch:        $(git rev-parse --abbrev-ref HEAD)"
+echo "HEAD:          $(git log -1 --oneline)"
+echo
+echo "Recent commits on lib/js/ and plans/:"
+git log -10 --oneline -- lib/js/ plans/js-on-sx.md 2>/dev/null || echo "  (none yet)"
+echo
+echo "=== Test baseline ==="
+if [ -x hosts/ocaml/_build/default/bin/sx_server.exe ]; then
+  echo -n "Unit tests:     "; bash lib/js/test.sh 2>&1 | tail -1 || true
+  echo -n "Conformance:    "; bash lib/js/conformance.sh 2>&1 | tail -1 || true
+else
+  echo "sx_server.exe not built — run ./scripts/sx-build-all.sh first."
+fi
+echo
+echo "=== test262 scoreboard ==="
+if [ -f lib/js/test262-scoreboard.json ]; then
+  echo "  ✓ lib/js/test262-scoreboard.json exists"
+  python3 -c "import json;d=json.load(open('lib/js/test262-scoreboard.json'));t=d.get('totals', d.get('overall', {}));print(f\"  totals: {t}\")" 2>/dev/null || true
+else
+  echo "  ✗ lib/js/test262-scoreboard.json NOT found"
+fi
+echo
+echo "=== regex platform hook ==="
+if grep -q js-regex-platform-override lib/js/runtime.sx 2>/dev/null; then
+  echo "  ✓ js-regex-platform-override! wired in runtime.sx"
+else
+  echo "  ✗ js-regex-platform-override! not yet wired"
+fi
+echo
+echo "=== Briefing ==="
+for f in plans/agent-briefings/*.md; do
+  [ -f "$f" ] && echo "  $f"
+done
+echo
+echo "To respawn: paste plans/agent-briefings/loop.md into Claude via the Agent tool"
+echo "            with run_in_background=true. The agent reads the plan's progress"
+echo "            log and picks up wherever the queue left off."
+
+if [ "${1:-}" = "--print" ]; then
+  echo
+  echo "=== Briefing contents ==="
+  for f in plans/agent-briefings/*.md; do
+    echo
+    echo "---- $f ----"
+    cat "$f"
+  done
+fi