VM: fix nested IO suspension frame corruption, island hydration preload

VM frame merging bug: call_closure_reuse now saves caller continuations
on a reuse_stack instead of merging frames. resume_vm restores them in
innermost-first order. Fixes frame count corruption when nested closures
suspend via OP_PERFORM. Zero test regressions (3924/3924).

Island hydration: hydrate-island now looks up components from (global-env)
instead of render-env, triggering the symbol resolve hook. Added JS-level
preload-island-defs that scans DOM for data-sx-island and loads definitions
from the content-addressed manifest BEFORE hydration — avoids K.load
reentrancy when the resolve hook fires inside env_get.

loadDefinitionByHash: fixed isMultiDefine check — defcomp/defisland bodies
containing nested (define ...) forms no longer suppress name insertion.
Added K.load return value checking for silent error string returns.

sx_browser.ml: resolve hook falls back to global_env.bindings when
_vm_globals miss (sync gap). Snapshot reuse_stack alongside pending_cek.

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

hosts/ocaml/browser/sx_browser.ml

@@ -269,7 +269,9 @@ let () =
       (* Check if the symbol appeared in globals after the load *)
       match Hashtbl.find_opt _vm_globals name with
       | Some v -> Some v
-      | None -> None)
+      | None ->
+        (* Fallback: check global_env directly if vm_globals missed the sync *)
+        Hashtbl.find_opt global_env.bindings (Sx_types.intern name))
 
 (* ================================================================== *)
 (* Core API                                                            *)