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.

lib/js/test262-scoreboard.md

@@ -1,15 +1,15 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 420.4s
+Wall time: 353.1s
 
-**Total:** 65/99 runnable passed (65.7%). Raw: pass=65 fail=26 skip=1 timeout=8 total=100.
+**Total:** 65/99 runnable passed (65.7%). Raw: pass=65 fail=28 skip=1 timeout=6 total=100.
 
 ## Top failure modes
 
 - **16x** Test262Error (assertion failed)
-- **8x** Timeout
-- **6x** TypeError: not a function
+- **8x** TypeError: not a function
+- **6x** Timeout
 - **2x** Unhandled: Not callable: \\\
 - **1x** ReferenceError (undefined symbol)
 - **1x** SyntaxError (parse/unsupported syntax)
@@ -18,14 +18,14 @@ Wall time: 420.4s
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/String | 65 | 26 | 1 | 8 | 100 | 65.7% |
+| built-ins/String | 65 | 28 | 1 | 6 | 100 | 65.7% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
 ### built-ins/String  (65/99 — 65.7%)
 
 - **16x** Test262Error (assertion failed)
-- **8x** Timeout
-- **6x** TypeError: not a function
+- **8x** TypeError: not a function
+- **6x** Timeout
 - **2x** Unhandled: Not callable: \\\
 - **1x** ReferenceError (undefined symbol)