VM: VmClosure value type + iterative run loop + define hoisting + SSR fixes

Core VM changes:
- Add VmClosure value variant — inner closures created by OP_CLOSURE are
  first-class VM values, not NativeFn wrappers around call_closure
- Convert `run` from recursive to while-loop — zero OCaml stack growth,
  true TCO for VmClosure tail calls
- vm_call handles VmClosure by pushing frame on current VM (no new VM
  allocation per call)
- Forward ref _vm_call_closure_ref for cross-boundary calls (CEK/primitives)

Compiler (spec/compiler.sx):
- Define hoisting in compile-begin: pre-allocate local slots for all
  define forms before compiling any values. Fixes forward references
  between inner functions (e.g. read-expr referencing skip-ws in sx-parse)
- scope-define-local made idempotent (skip if slot already exists)

Server (sx_server.ml):
- JIT fail-once sentinel: mark l_compiled as failed after first VM runtime
  error. Eliminates thousands of retry attempts per page render.
- HTML tag bindings: register all HTML tags as pass-through NativeFns so
  eval-expr can handle (div ...) etc. in island component bodies.
- Log VM FAIL errors with function name before disabling JIT.

SSR fixes:
- adapter-html.sx letrec handler: evaluate bindings in proper letrec scope
  (pre-bind nil, then evaluate), render body with render-to-html instead of
  eval-expr. Fixes island SSR for components using letrec.
- Add `init` primitive to OCaml kernel (all-but-last of list).
- VmClosure handling in sx_runtime.ml sx_call dispatch.

Tests: 971/971 OCaml (+19 new), 0 failures.

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

hosts/ocaml/lib/sx_primitives.ml

@@ -333,6 +333,11 @@ let () =
     | [List l] | [ListRef { contents = l }] ->
       (match List.rev l with x :: _ -> x | [] -> Nil)
     | _ -> raise (Eval_error "last: 1 list arg"));
+  register "init" (fun args ->
+    match args with
+    | [List l] | [ListRef { contents = l }] ->
+      (match List.rev l with _ :: rest -> List (List.rev rest) | [] -> List [])
+    | _ -> raise (Eval_error "init: 1 list arg"));
   register "nth" (fun args ->
     match args with
     | [List l; Number n] | [ListRef { contents = l }; Number n] ->