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/transpile.sx

@@ -23,7 +23,46 @@
 
 ;; ── tiny helpers ──────────────────────────────────────────────────
 
-(define js-sym (fn (name) (make-symbol name)))
+(define js-has-dollar? (fn (name) (js-has-dollar-loop? name 0 (len name))))
+(define
+  js-has-dollar-loop?
+  (fn
+    (s i n)
+    (cond
+      ((>= i n) false)
+      ((= (char-at s i) "$") true)
+      (else (js-has-dollar-loop? s (+ i 1) n)))))
+
+(define
+  js-mangle-ident
+  (fn
+    (name)
+    (if
+      (js-has-dollar? name)
+      (js-mangle-ident-loop name 0 (len name) "")
+      name)))
+
+;; ── main dispatcher ───────────────────────────────────────────────
+
+(define
+  js-mangle-ident-loop
+  (fn
+    (s i n acc)
+    (cond
+      ((>= i n) acc)
+      ((= (char-at s i) "$")
+        (js-mangle-ident-loop s (+ i 1) n (str acc "_js_dollar_")))
+      (else (js-mangle-ident-loop s (+ i 1) n (str acc (char-at s i)))))))
+
+;; ── Identifier lookup ─────────────────────────────────────────────
+
+;; `undefined` in JS is really a global binding. If the parser emits
+;; (js-undef) we handle that above. A bare `undefined` ident also maps
+;; to the same sentinel.
+(define js-sym (fn (name) (make-symbol (js-mangle-ident name))))
+
+;; ── Unary ops ─────────────────────────────────────────────────────
+
 (define
   js-tag?
   (fn
@@ -34,9 +73,11 @@
       (= (type-of (first ast)) "symbol")
       (= (symbol-name (first ast)) tag))))
 
+;; ── Binary ops ────────────────────────────────────────────────────
+
 (define js-ast-tag (fn (ast) (symbol-name (first ast))))
 
-;; ── main dispatcher ───────────────────────────────────────────────
+;; ── Member / index ────────────────────────────────────────────────
 
 (define
   js-transpile
@@ -146,11 +187,6 @@
       (else
         (error (str "js-transpile: unexpected value type: " (type-of ast)))))))
 
-;; ── Identifier lookup ─────────────────────────────────────────────
-
-;; `undefined` in JS is really a global binding. If the parser emits
-;; (js-undef) we handle that above. A bare `undefined` ident also maps
-;; to the same sentinel.
 (define
   js-transpile-ident
   (fn
@@ -164,8 +200,10 @@
       ((= name "Function") (js-sym "js-function-global"))
       (else (js-sym name)))))
 
-;; ── Unary ops ─────────────────────────────────────────────────────
+;; ── Call ──────────────────────────────────────────────────────────
 
+;; JS `f(a, b, c)` → `(f a b c)` after transpile. Works for both
+;; identifier calls and computed callee (arrow fn, member access).
 (define
   js-transpile-unop
   (fn
@@ -196,7 +234,7 @@
             ((= op "void") (list (js-sym "quote") :js-undefined))
             (else (error (str "js-transpile-unop: unsupported op: " op)))))))))
 
-;; ── Binary ops ────────────────────────────────────────────────────
+;; ── Array literal ─────────────────────────────────────────────────
 
 (define
   js-transpile-binop
@@ -259,22 +297,25 @@
             (js-sym "_a"))))
       (else (error (str "js-transpile-binop: unsupported op: " op))))))
 
-;; ── Member / index ────────────────────────────────────────────────
+;; ── Object literal ────────────────────────────────────────────────
 
+;; Build a dict by `(dict)` + `dict-set!` inside a `let` that yields
+;; the dict as its final expression. This keeps keys in JS insertion
+;; order and allows computed values.
 (define
   js-transpile-member
   (fn (obj key) (list (js-sym "js-get-prop") (js-transpile obj) key)))
 
+;; ── Conditional ───────────────────────────────────────────────────
+
 (define
   js-transpile-index
   (fn
     (obj idx)
     (list (js-sym "js-get-prop") (js-transpile obj) (js-transpile idx))))
 
-;; ── Call ──────────────────────────────────────────────────────────
+;; ── Arrow function ────────────────────────────────────────────────
 
-;; JS `f(a, b, c)` → `(f a b c)` after transpile. Works for both
-;; identifier calls and computed callee (arrow fn, member access).
 (define
   js-transpile-call
   (fn
@@ -320,8 +361,11 @@
           (js-transpile callee)
           (js-transpile-args args))))))
 
-;; ── Array literal ─────────────────────────────────────────────────
+;; ── Assignment ────────────────────────────────────────────────────
 
+;; `a = b` on an ident → (set! a b).
+;; `a += b` on an ident → (set! a (js-add a b)).
+;; `obj.k = v` / `obj[k] = v` → (js-set-prop obj "k" v).
 (define
   js-transpile-new
   (fn
@@ -331,11 +375,6 @@
       (js-transpile callee)
       (cons (js-sym "list") (map js-transpile args)))))
 
-;; ── Object literal ────────────────────────────────────────────────
-
-;; Build a dict by `(dict)` + `dict-set!` inside a `let` that yields
-;; the dict as its final expression. This keeps keys in JS insertion
-;; order and allows computed values.
 (define
   js-transpile-array
   (fn
@@ -354,8 +393,6 @@
           elts))
       (cons (js-sym "list") (map js-transpile elts)))))
 
-;; ── Conditional ───────────────────────────────────────────────────
-
 (define
   js-has-spread?
   (fn
@@ -365,8 +402,9 @@
       ((js-tag? (first lst) "js-spread") true)
       (else (js-has-spread? (rest lst))))))
 
-;; ── Arrow function ────────────────────────────────────────────────
+;; ── End-to-end entry points ───────────────────────────────────────
 
+;; Transpile + eval a single JS expression string.
 (define
   js-transpile-args
   (fn
@@ -385,11 +423,8 @@
           args))
       (cons (js-sym "list") (map js-transpile args)))))
 
-;; ── Assignment ────────────────────────────────────────────────────
-
-;; `a = b` on an ident → (set! a b).
-;; `a += b` on an ident → (set! a (js-add a b)).
-;; `obj.k = v` / `obj[k] = v` → (js-set-prop obj "k" v).
+;; Transpile a JS expression string to SX source text (for inspection
+;; in tests). Useful for asserting the exact emitted tree.
 (define
   js-transpile-object
   (fn
@@ -451,9 +486,6 @@
                 (append inits (list (js-transpile body))))))))
       (list (js-sym "fn") param-syms body-tr))))
 
-;; ── End-to-end entry points ───────────────────────────────────────
-
-;; Transpile + eval a single JS expression string.
 (define
   js-transpile-tpl
   (fn
@@ -465,8 +497,6 @@
       (else
         (cons (js-sym "js-template-concat") (js-transpile-tpl-parts parts))))))
 
-;; Transpile a JS expression string to SX source text (for inspection
-;; in tests). Useful for asserting the exact emitted tree.
 (define
   js-transpile-tpl-parts
   (fn