Fix 73 JS test failures: match transpiler, sxEq, deref frame, signals, stepper lib

Evaluator fixes (from broken match refactor in 8bba02f):
- Deref frame: use CEK state `value`, not `(get frame "value")`
- Deref frame: restore `(context "sx-reactive" nil)` (was undefined `get-tracking-context`)
- Scope-acc frame: restore missing `(get frame "value")` arg to make-scope-acc-frame
- Add missing `thread-insert-arg` helper for thread-first non-HO branch

Transpiler (hosts/javascript/transpiler.sx):
- Add `match` special form handler (IIFE with chained if/return, `_` wildcard)
- Replace `=`/`!=` infix `==` with `sxEq()` function call for proper symbol equality

JS platform (hosts/javascript/platform.py):
- Add `sxEq` for structural symbol/keyword comparison
- Add `componentFile`, `sort`, `defStore`/`useStore`/`clearStores` primitives
- Add `length`/`map`/`for-each`/`reduce` as VM-compatible HOF primitives
- Fix `SYM` → `makeSymbol` references

New files:
- sx/sx/stepper-lib.sx: extracted split-tag, build-code-tokens, steps-to-preview

JS tests: 0 → 1582/1585 passing (3 remaining are VM closure interop)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/javascript/platform.py

@@ -835,6 +835,16 @@ PREAMBLE = '''\
 ;(function(global) {
   "use strict";
 
+  // =========================================================================
+  // Equality — used by transpiled code (= a b) → sxEq(a, b)
+  // =========================================================================
+  function sxEq(a, b) {
+    if (a === b) return true;
+    if (a && b && a._sym && b._sym) return a.name === b.name;
+    if (a && b && a._kw && b._kw) return a.name === b.name;
+    return false;
+  }
+
   // =========================================================================
   // Types
   // =========================================================================
@@ -944,8 +954,8 @@ PRIMITIVES_JS_MODULES: dict[str, str] = {
 
     "core.comparison": '''
   // core.comparison
-  PRIMITIVES["="] = function(a, b) { return a === b; };
-  PRIMITIVES["!="] = function(a, b) { return a !== b; };
+  PRIMITIVES["="] = sxEq;
+  PRIMITIVES["!="] = function(a, b) { return !sxEq(a, b); };
   PRIMITIVES["<"] = function(a, b) { return a < b; };
   PRIMITIVES[">"] = function(a, b) { return a > b; };
   PRIMITIVES["<="] = function(a, b) { return a <= b; };
@@ -1306,6 +1316,7 @@ PLATFORM_JS_PRE = '''
   function componentClosure(c) { return c.closure; }
   function componentHasChildren(c) { return c.hasChildren; }
   function componentName(c) { return c.name; }
+  function componentFile(c) { return (c && c.file) ? c.file : NIL; }
   function componentAffinity(c) { return c.affinity || "auto"; }
   function componentParamTypes(c) { return (c && c._paramTypes) ? c._paramTypes : NIL; }
   function componentSetParamTypes_b(c, t) { if (c) c._paramTypes = t; return NIL; }
@@ -2669,12 +2680,12 @@ PLATFORM_ORCHESTRATION_JS = """
   function cekTry(thunkFn, handlerFn) {
     try {
       var result = _wrapSxFn(thunkFn)();
-      if (!handlerFn || handlerFn === NIL) return [SYM("ok"), result];
+      if (!handlerFn || handlerFn === NIL) return [makeSymbol("ok"), result];
       return result;
     } catch (e) {
       var msg = (e && e.message) ? e.message : String(e);
       if (handlerFn && handlerFn !== NIL) return _wrapSxFn(handlerFn)(msg);
-      return [SYM("error"), msg];
+      return [makeSymbol("error"), msg];
     }
   }
   function errorMessage(e) {
@@ -3223,6 +3234,43 @@ def fixups_js(has_html, has_sx, has_dom, has_signals=False, has_deps=False, has_
   // Core primitives that require native JS (cannot be expressed via FFI)
   // -----------------------------------------------------------------------
   PRIMITIVES["error"] = function(msg) { throw new Error(msg); };
+  PRIMITIVES["sort"] = function(lst) {
+    if (!Array.isArray(lst)) return lst;
+    return lst.slice().sort(function(a, b) {
+      if (a < b) return -1; if (a > b) return 1; return 0;
+    });
+  };
+
+  // Aliases for VM bytecode compatibility
+  PRIMITIVES["length"] = PRIMITIVES["len"];
+  // VM-compatible HOF primitives — use callPrimFn which handles native, lambda, and VM closures
+  function callPrimFn(f, args) {
+    if (typeof f === "function") return f.apply(null, args);
+    if (f && f._lambda) return cekCall(f, args);
+    if (f && f["vm-code"]) {
+      // VM closure — call through call-primitive dispatch
+      var cp = PRIMITIVES["vm-call-closure"];
+      if (cp) return cp(f, args);
+    }
+    return cekCall(f, args);
+  }
+  PRIMITIVES["map"] = function(fn, lst) {
+    if (Array.isArray(fn)) { var tmp = fn; fn = lst; lst = tmp; }
+    var result = [];
+    for (var i = 0; i < lst.length; i++) result.push(callPrimFn(fn, [lst[i]]));
+    return result;
+  };
+  PRIMITIVES["for-each"] = function(fn, lst) {
+    if (Array.isArray(fn)) { var tmp = fn; fn = lst; lst = tmp; }
+    for (var i = 0; i < lst.length; i++) callPrimFn(fn, [lst[i]]);
+    return NIL;
+  };
+  PRIMITIVES["reduce"] = function(fn, init, lst) {
+    if (Array.isArray(fn)) { var tmp = fn; fn = lst; lst = init; init = tmp; }
+    var acc = init;
+    for (var i = 0; i < lst.length; i++) acc = callPrimFn(fn, [acc, lst[i]]);
+    return acc;
+  };
 
   // FFI library functions — defined in dom.sx/browser.sx but not transpiled.
   // Registered here so runtime-evaluated SX code (data-init, islands) can use them.
@@ -3303,14 +3351,30 @@ def fixups_js(has_html, has_sx, has_dom, has_signals=False, has_deps=False, has_
   PRIMITIVES["cek-try"] = function(thunkFn, handlerFn) {
     try {
       var result = _wrapSxFn(thunkFn)();
-      if (!handlerFn || handlerFn === NIL) return [SYM("ok"), result];
+      if (!handlerFn || handlerFn === NIL) return [makeSymbol("ok"), result];
       return result;
     } catch (e) {
       var msg = (e && e.message) ? e.message : String(e);
       if (handlerFn && handlerFn !== NIL) return _wrapSxFn(handlerFn)(msg);
-      return [SYM("error"), msg];
+      return [makeSymbol("error"), msg];
     }
-  };''']
+  };
+  // Named stores — global mutable registry (mirrors OCaml sx_primitives.ml)
+  var _storeRegistry = {};
+  function defStore(name, initFn) {
+    if (!_storeRegistry.hasOwnProperty(name)) {
+      _storeRegistry[name] = _wrapSxFn(initFn)();
+    }
+    return _storeRegistry[name];
+  }
+  function useStore(name) {
+    if (!_storeRegistry.hasOwnProperty(name)) throw new Error("Store not found: " + name);
+    return _storeRegistry[name];
+  }
+  function clearStores() { _storeRegistry = {}; return NIL; }
+  PRIMITIVES["def-store"] = defStore;
+  PRIMITIVES["use-store"] = useStore;
+  PRIMITIVES["clear-stores"] = clearStores;''']
     if has_deps:
         lines.append('''
   // Platform deps functions (native JS, not transpiled — need explicit registration)