js-on-sx: lexer handles \uXXXX and \xXX string escapes

read-string fell through to the literal-char branch for \u and \x,
silently stripping the backslash ("A".length returned 5 instead
of 1). Added js-hex-value helper and two cond clauses that read the
hex digits via js-peek + js-hex-digit?, compute the code point, and
emit it via char-from-code. Invalid escapes fall through to the
literal-char behaviour. built-ins/String (with --restart-every 1):
65/99 → 68/99. conformance.sh: 148/148.

plans/js-on-sx.md

@@ -158,6 +158,8 @@ Each item: implement → tests → update progress. Mark `[x]` when tests green.
 
 Append-only record of completed iterations. Loop writes one line per iteration: date, what was done, test count delta.
 
+- 2026-05-07 — **JS lexer: handle `\uXXXX` and `\xXX` escape sequences in string literals.** The `read-string` cond fell through to the literal-char branch for `\u` and `\x`, silently stripping the backslash (so `"A".length` returned 5 instead of 1). Added `js-hex-value` helper and two new cond clauses that read the hex digits via `js-peek` + `js-hex-digit?`, compute the code point, and emit it via `char-from-code`. Invalid escapes (no following hex digits) fall through to the literal-char behaviour for compatibility. With test isolation (`--restart-every 1`) built-ins/String 65/99 → 68/99. Without isolation the headline stays at 65/99 because state pollution between sibling tests dominates. conformance.sh: 148/148.
+
 - 2026-05-07 — **Bump test262 runner default per-test timeout 5s→15s.** With 4 parallel workers contending for CPU, the 5s default was timing out the vast majority of tests (e.g. 85/99 on built-ins/String). Direct invocation showed individual tests complete in ~3s, but parallel scheduling stretched wall time to >5s. Bumping to 15s makes the scoreboard usable: built-ins/String 14.1% → 65.7% (65/99), with real failure modes now visible (16x Test262Error, 6x TypeError, etc.) instead of "85x Timeout" drowning the signal. Regenerated scoreboard to reflect the new state. conformance.sh: 148/148.
 
 - 2026-05-06 — **Fix rational-zero-division regression in core JS constants + charCodeAt missing primitives.** OCaml binary uses rationals for integer literals, so `(/ 0 0)` and `(/ 1 0)` throw "rational: division by zero" instead of producing NaN/Infinity. Replaced `(/ 0 0)` → `nan` (`js-nan-value`); `(/ 1 0)` → `inf` (`js-infinity-value`, `js-math-min` empty case, `js-number-is-finite`); `(- 0 (/ 1 0))` → `-inf` (`js-math-max` empty case); `(/ -1 0)` → `-inf` (`js-number-is-finite`). `js-max-value-approx` was looping forever (rationals never reach float infinity) — replaced with literal `1.7976931348623157e+308`. Fixed `charCodeAt` and string `.length` to use `(len s)` and `(char-code (char-at s idx))` instead of missing `unicode-len`/`unicode-char-code-at` primitives. conformance.sh: 0→148/148. Unit tests: 521/530 best run (baseline run was 417/530; both timeout-flaky).