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>

shared/static/wasm/sx_browser.bc.wasm.js

@@ -1792,7 +1792,7 @@
             blake2_js_for_wasm_create: blake2_js_for_wasm_create};
    }
    (globalThis))
-({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["sx-eb076217",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-36a151d2",[2,3,5]],["std_exit-10fb8830",[2]],["start-80fdb768",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["sx-eb076217",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-87cfa374",[2,3,5]],["std_exit-10fb8830",[2]],["start-80fdb768",0]],"generated":(b=>{var
 c=b,a=b?.module?.export||b;return{"env":{"caml_ba_kind_of_typed_array":()=>{throw new
 Error("caml_ba_kind_of_typed_array not implemented")},"caml_exn_with_js_backtrace":()=>{throw new
 Error("caml_exn_with_js_backtrace not implemented")},"caml_int64_create_lo_mi_hi":()=>{throw new