js-on-sx: exponent notation in js-string-to-number (+3 Number tests)

js-num-from-string now finds an e/E split, parses mantissa and exponent
separately, and combines via js-pow-int (positive-exp loop for >=0, 1/
reciprocal for negative). Previously `.12345e-3` parsed as 0.12345 and
"1e3" returned NaN — the parser walked decimals/dots only.

New helpers:
- js-find-exp-char / -loop : linear scan for e/E, returns -1 if absent
- js-pow-int base exp : integer-exp power, handles negative

Also fixed `js-string-trim` typo → `js-trim` in the rewritten num-from-
string, and corrected test 903's expected part count (3, not 2 — the
lexer has always split `hi ${x}!` into str+expr+str, the test just had
the wrong count).

Unit: 521/522 (was 520/522, 934 still blocked on SX \` escape).
Conformance: 148/148 unchanged.
Number scoreboard: 43/100 → 46/100 (+3).

Impacted test262 paths (sample): built-ins/Number/S9.3.1_A11.js and
A12/A16/A17 (".12345e-3", scientific notation round-trips).

lib/js/test262-runner.py

@@ -1120,6 +1120,8 @@ def main(argv):
     ap.add_argument("--output-md", type=str,
                     default=str(REPO / "lib" / "js" / "test262-scoreboard.md"))
     ap.add_argument("--progress-every", type=int, default=100)
+    ap.add_argument("--dump-failures", type=str, default=None,
+                    help="if set, write every failed test's rel path + reason to this file")
     args = ap.parse_args(argv)
 
     if not SX_SERVER.exists():
@@ -1242,6 +1244,15 @@ def main(argv):
     out_md = Path(args.output_md)
     write_markdown(scoreboard, out_md, pinned_commit, t_run_elapsed)
 
+    if args.dump_failures:
+        out_fail = Path(args.dump_failures)
+        out_fail.parent.mkdir(parents=True, exist_ok=True)
+        with out_fail.open("w", encoding="utf-8") as f:
+            for r in results:
+                if r.status in ("fail", "timeout"):
+                    f.write(f"{r.status}\t{r.rel}\t{r.reason}\n")
+        print(f"failures dumped to {out_fail}", file=sys.stderr)
+
     t = scoreboard["totals"]
     print(
         f"\nScoreboard: {t['pass']}/{t['runnable']} runnable passed ({t['pass_rate']}%)  "