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>

spec/evaluator.sx

@@ -42,6 +42,17 @@
 
 (define make-thread-frame (fn (remaining env) {:env env :type "thread" :remaining remaining}))
 
+(define
+  thread-insert-arg
+  (fn
+    (form value fenv)
+    (if
+      (= (type-of form) "list")
+      (eval-expr
+        (cons (first form) (cons (list (quote quote) value) (rest form)))
+        fenv)
+      (eval-expr (list form (list (quote quote) value)) fenv))))
+
 (define make-map-frame (fn (f remaining results env) {:indexed false :env env :results results :type "map" :f f :remaining remaining}))
 
 (define make-map-indexed-frame (fn (f remaining results env) {:indexed true :env env :results results :type "map" :f f :remaining remaining}))
@@ -2161,7 +2172,7 @@
             ("reset" (make-cek-value value env rest-k))
             ("deref"
               (let
-                ((val (get frame "value")) (fenv (get frame "env")))
+                ((val value) (fenv (get frame "env")))
                 (if
                   (not (signal? val))
                   (make-cek-value val fenv rest-k)
@@ -2170,7 +2181,7 @@
                     (reactive-shift-deref val fenv rest-k)
                     (do
                       (let
-                        ((ctx (get-tracking-context)))
+                        ((ctx (context "sx-reactive" nil)))
                         (when
                           ctx
                           (let
@@ -2232,7 +2243,7 @@
                     fenv
                     (kont-push
                       (let
-                        ((new-frame (make-scope-acc-frame (get frame "name") (rest remaining) fenv)))
+                        ((new-frame (make-scope-acc-frame (get frame "name") (get frame "value") (rest remaining) fenv)))
                         (dict-set! new-frame "emitted" (get frame "emitted"))
                         new-frame)
                       rest-k)))))