js-on-sx: harness cache — precompute HARNESS_STUB SX once per run

Root cause: every sx_server worker session used js-eval on the 3.6KB
HARNESS_STUB, paying ~15s for tokenize+parse+transpile even though every
session does the same thing. Over a full scoreboard with periodic worker
restarts that's minutes of wasted work.

Fix: transpile once per Python process. Spin up a throwaway sx_server,
run (inspect (js-transpile (js-parse (js-tokenize HARNESS_STUB)))), write
the resulting SX source to lib/js/.harness-cache/stub.<fingerprint>.sx and
a stable-name symlink-ish copy stub.sx. Every worker session then does a
single (load .harness-cache/stub.sx) instead of re-running js-eval.

Fingerprint: sha256(HARNESS_STUB + lexer.sx + parser.sx + transpile.sx).
Transpiler edits invalidate the cache automatically. Runs back-to-back
reuse the cache — only the first run after a transpiler change pays the
~15s precompute.

Transpile had to gain a $-to-_js_dollar_ name-mangler: the SX tokenizer
rejects $ in identifiers, which broke round-tripping via inspect. JS
$DONOTEVALUATE → SX _js_dollar_DONOTEVALUATE. Internal JS-on-SX names are
unaffected (none contain $).

Measured: 300-test wide (Math+Number+String @ 100/cat, --per-test-timeout 5):
593.7s → 288.0s, 2.06x speedup. Scoreboard 114→115/300 (38.3%, noise band).
Math 40%, Number 44%, String 30% — same shape as prior.

Baselines: 520/522 unit, 148/148 slice — unchanged.

lib/js/test262-scoreboard.json

@@ -1,12 +1,12 @@
 {
   "totals": {
-    "pass": 118,
-    "fail": 160,
+    "pass": 115,
+    "fail": 174,
     "skip": 1597,
-    "timeout": 22,
+    "timeout": 11,
     "total": 1897,
     "runnable": 300,
-    "pass_rate": 39.3
+    "pass_rate": 38.3
   },
   "categories": [
     {
@@ -35,46 +35,42 @@
     {
       "category": "built-ins/Number",
       "total": 340,
-      "pass": 48,
-      "fail": 45,
+      "pass": 44,
+      "fail": 52,
       "skip": 240,
-      "timeout": 7,
-      "pass_rate": 48.0,
+      "timeout": 4,
+      "pass_rate": 44.0,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          42
+          52
         ],
         [
           "Timeout",
-          7
-        ],
-        [
-          "ReferenceError (undefined symbol)",
-          3
+          4
         ]
       ]
     },
     {
       "category": "built-ins/String",
       "total": 1223,
-      "pass": 30,
-      "fail": 56,
+      "pass": 31,
+      "fail": 63,
       "skip": 1123,
-      "timeout": 14,
-      "pass_rate": 30.0,
+      "timeout": 6,
+      "pass_rate": 31.0,
       "top_failures": [
         [
           "Test262Error (assertion failed)",
-          43
+          53
         ],
         [
           "Timeout",
-          14
+          6
         ],
         [
           "ReferenceError (undefined symbol)",
-          7
+          2
         ],
         [
           "Unhandled: Not callable: \\\\\\",
@@ -100,19 +96,19 @@
   "top_failure_modes": [
     [
       "Test262Error (assertion failed)",
-      108
+      128
     ],
     [
       "TypeError: not a function",
-      36
+      37
     ],
     [
       "Timeout",
-      22
+      11
     ],
     [
       "ReferenceError (undefined symbol)",
-      10
+      2
     ],
     [
       "Unhandled: Not callable: \\\\\\",
@@ -129,9 +125,13 @@
     [
       "Unhandled: Not callable: {:__proto__ {}} (kont=5 frames)\\",
       1
+    ],
+    [
+      "Unhandled: js-transpile-binop: unsupported op: >>>\\",
+      1
     ]
   ],
   "pinned_commit": "d5e73fc8d2c663554fb72e2380a8c2bc1a318a33",
-  "elapsed_seconds": 429.3,
+  "elapsed_seconds": 288.0,
   "workers": 1
 }