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.md

@@ -1,36 +1,37 @@
 # test262 scoreboard
 
 Pinned commit: `d5e73fc8d2c663554fb72e2380a8c2bc1a318a33`
-Wall time: 429.3s
+Wall time: 288.0s
 
-**Total:** 118/300 runnable passed (39.3%). Raw: pass=118 fail=160 skip=1597 timeout=22 total=1897.
+**Total:** 115/300 runnable passed (38.3%). Raw: pass=115 fail=174 skip=1597 timeout=11 total=1897.
 
 ## Top failure modes
 
-- **108x** Test262Error (assertion failed)
-- **36x** TypeError: not a function
-- **22x** Timeout
-- **10x** ReferenceError (undefined symbol)
+- **128x** Test262Error (assertion failed)
+- **37x** TypeError: not a function
+- **11x** Timeout
+- **2x** ReferenceError (undefined symbol)
 - **2x** Unhandled: Not callable: \\\
 - **2x** Unhandled: Not callable: {:__proto__ {}} (kont=6 frames)\
 - **1x** SyntaxError (parse/unsupported syntax)
 - **1x** Unhandled: Not callable: {:__proto__ {}} (kont=5 frames)\
+- **1x** Unhandled: js-transpile-binop: unsupported op: >>>\
 
 ## Categories (worst pass-rate first, min 10 runnable)
 
 | Category | Pass | Fail | Skip | Timeout | Total | Pass % |
 |---|---:|---:|---:|---:|---:|---:|
-| built-ins/String | 30 | 56 | 1123 | 14 | 1223 | 30.0% |
+| built-ins/String | 31 | 63 | 1123 | 6 | 1223 | 31.0% |
 | built-ins/Math | 40 | 59 | 227 | 1 | 327 | 40.0% |
-| built-ins/Number | 48 | 45 | 240 | 7 | 340 | 48.0% |
+| built-ins/Number | 44 | 52 | 240 | 4 | 340 | 44.0% |
 
 ## Per-category top failures (min 10 runnable, worst first)
 
-### built-ins/String  (30/100 — 30.0%)
+### built-ins/String  (31/100 — 31.0%)
 
-- **43x** Test262Error (assertion failed)
-- **14x** Timeout
-- **7x** ReferenceError (undefined symbol)
+- **53x** Test262Error (assertion failed)
+- **6x** Timeout
+- **2x** ReferenceError (undefined symbol)
 - **2x** Unhandled: Not callable: \\\
 - **2x** Unhandled: Not callable: {:__proto__ {}} (kont=6 frames)\
 
@@ -40,8 +41,7 @@ Wall time: 429.3s
 - **23x** Test262Error (assertion failed)
 - **1x** Timeout
 
-### built-ins/Number  (48/100 — 48.0%)
+### built-ins/Number  (44/100 — 44.0%)
 
-- **42x** Test262Error (assertion failed)
-- **7x** Timeout
-- **3x** ReferenceError (undefined symbol)
+- **52x** Test262Error (assertion failed)
+- **4x** Timeout