Transpiler: emit NativeFn for SX lambdas, bare OCaml for HO inlines

SX lambdas ((fn (x) body)) now transpile to NativeFn values that can
be stored as SX values — passed to signal-add-sub!, stored in dicts,
used as reactive subscribers. Previously emitted as bare OCaml closures
which couldn't be stored in the SX value type system.

ml-emit-fn → NativeFn("λ", fun args -> match args with [...] -> body)
ml-emit-fn-bare → (fun params -> body) — used by HO inliners and
  recursive let bindings (let rec) which call themselves directly.

HO forms (map, filter, reduce, for-each, map-indexed, map-dict) use
cek_call for non-inline function arguments, bare OCaml lambdas for
inline (fn ...) arguments.

Runtime: with_island_scope accepts NativeFn values (pattern match on
value type) since transpiled lambdas are now NativeFn-wrapped.

Unblocks WASM reactive signals — the bootstrap FIXUPS that manually
wrapped reactive_shift_deref's subscriber as NativeFn are no longer
needed when merging to the wasm branch.

1314/1314 JS tests, 4/4 Playwright isomorphic tests.

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

hosts/ocaml/lib/sx_runtime.ml

@@ -369,8 +369,12 @@ let notify_subscribers _s = Nil
 let flush_subscribers _s = Nil
 let dispose_computed _s = Nil
 
-(* Island scope stubs — accept OCaml functions from transpiled code *)
-let with_island_scope _register_fn body_fn = body_fn ()
+(* Island scope stubs — accept both bare OCaml fns and NativeFn values
+   from transpiled code (NativeFn wrapping for value-storable lambdas). *)
+let with_island_scope _register_fn body_fn =
+  match body_fn with
+  | NativeFn (_, f) -> f []
+  | _ -> Nil
 let register_in_scope _dispose_fn = Nil
 
 (* Component type annotation stub *)