Fix WASM reactive signals: unify context/scope, fix flush-subscribers

Three root causes for reactive attribute updates not propagating in WASM:

1. `context` CEK special form only searched kont provide frames, missing
   `scope-push!` entries in the native scope_stacks hashtable. Unified by
   adding scope_stacks fallback to step_sf_context.

2. `flush-subscribers` used bare `(sub)` call which failed to invoke
   complex closures in for-each HO callbacks. Changed to `(cek-call sub nil)`.

3. Test eagerly evaluated `(deref s)` before render-to-dom saw it.
   Fixed tests to use quoted expressions matching real browser boot.

WASM native: 10/10, WASM shell: 26/26.

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

shared/static/wasm/sx/core-signals.sx

@@ -163,7 +163,7 @@
   :effects (mutation)
   (fn
     ((s :as dict))
-    (for-each (fn ((sub :as lambda)) (sub)) (signal-subscribers s))))
+    (for-each (fn (sub) (cek-call sub nil)) (signal-subscribers s))))
 
 (define
   dispose-computed

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-6003da22",[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
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["sx-a0c22109",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-8ae21d0a",[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