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.

lib/js/test262-scoreboard.md

@@ -0,0 +1,21 @@
+# test262 scoreboard
+
+Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
+
+**Total:** 0/8 passed (0.0%), 1 failed, 0 skipped, 7 timeouts.
+
+## Top failure modes
+
+- **1x** SyntaxError (parse/unsupported syntax)
+
+## Categories (worst pass-rate first)
+
+| Category | Pass | Fail | Skip | Timeout | Total | Pass % |
+|---|---:|---:|---:|---:|---:|---:|
+| built-ins/Math | 0 | 1 | 0 | 7 | 8 | 0.0% |
+
+## Per-category top failures
+
+### built-ins/Math
+
+- **1x** SyntaxError (parse/unsupported syntax)