js-on-sx: parallel test262 runner with raw-fd line buffer

Rework test262-runner.py to support --workers N parallel shards, each running
a long-lived sx_server session. Replace thread-per-readline with a select-based
raw-fd line buffer.

On 2-core machines, 1 worker still beats 2 (OCaml eval is CPU-bound and starves
when shared). Auto-defaults n_workers=1 on <=2 CPU, nproc-1 (up to 8) otherwise.

Throughput baseline: ~1.1 Math tests/s serial on 2-core (unchanged; the
evaluator dominates). The runner framework is now ready to scale on bigger
machines without further code changes.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/js/test262-scoreboard.json

@@ -1,22 +1,22 @@
 {
   "totals": {
-    "pass": 66,
-    "fail": 206,
+    "pass": 67,
+    "fail": 204,
     "skip": 39,
-    "timeout": 16,
+    "timeout": 17,
     "total": 327,
     "runnable": 288,
-    "pass_rate": 22.9
+    "pass_rate": 23.3
   },
   "categories": [
     {
       "category": "built-ins/Math",
       "total": 327,
-      "pass": 66,
-      "fail": 206,
+      "pass": 67,
+      "fail": 204,
       "skip": 39,
-      "timeout": 16,
-      "pass_rate": 22.9,
+      "timeout": 17,
+      "pass_rate": 23.3,
       "top_failures": [
         [
           "ReferenceError (undefined symbol)",
@@ -28,11 +28,11 @@
         ],
         [
           "TypeError: not a function",
-          31
+          30
         ],
         [
           "Timeout",
-          16
+          17
         ],
         [
           "Unhandled: Not callable: {:random <js-math-random()> :trunc <js-math-tr",
@@ -52,21 +52,18 @@
     ],
     [
       "TypeError: not a function",
-      31
+      30
     ],
     [
       "Timeout",
-      16
+      17
     ],
     [
       "Unhandled: Not callable: {:random <js-math-random()> :trunc <js-math-tr",
       1
-    ],
-    [
-      "SyntaxError (parse/unsupported syntax)",
-      1
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 275.0
+  "elapsed_seconds": 426.2,
+  "workers": 2
 }