vm-ext: phase A — extension dispatch fallthrough in sx_vm.ml

Adds Invalid_opcode of int exception and extension_dispatch_ref forward
ref (default raises Invalid_opcode op), plus the |op when op >= 200 arm
before the catch-all in the bytecode dispatch loop. Partition comment
documents 1-199 core / 200-247 extensions / 248-255 reserved.

Phase B will install the real registry's dispatch into the ref at module
init, replacing this stub.

Tests: 4 new foundation cases (Invalid_opcode for 200/224/247, Eval_error
for 199 to pin the threshold). +4 pass vs baseline, no regressions.

hosts/ocaml/lib/sx_vm.ml

@@ -44,6 +44,11 @@ type vm = {
     ip past OP_PERFORM, stack ready for a result push). *)
 exception VmSuspended of value * vm
 
+(** Raised by the extension dispatch fallthrough when an opcode in the
+    extension range (≥ 200) is encountered with no handler registered.
+    Carries the offending opcode id. See plans/sx-vm-opcode-extension.md. *)
+exception Invalid_opcode of int
+
 (* Register the VM suspension converter so sx_runtime.sx_apply_cek can
    catch VmSuspended and convert it to CekPerformRequest without a
    direct dependency on this module. *)
@@ -57,6 +62,14 @@ let () = Sx_types._convert_vm_suspension := (fun exn ->
 let jit_compile_ref : (lambda -> (string, value) Hashtbl.t -> vm_closure option) ref =
   ref (fun _ _ -> None)
 
+(** Forward reference for extension opcode dispatch — Phase B installs the
+    real registry's dispatch function here at module init. Until then, any
+    opcode in the extension range raises [Invalid_opcode]. Same forward-ref
+    pattern as [jit_compile_ref] above; keeps [Sx_vm_extensions] free to
+    depend on [Sx_vm]'s [vm] / [frame] types without a cycle. *)
+let extension_dispatch_ref : (int -> vm -> frame -> unit) ref =
+  ref (fun op _vm _frame -> raise (Invalid_opcode op))
+
 (* JIT threshold and counters live in Sx_types so primitives can read them
    without creating a sx_primitives → sx_vm dependency cycle. *)
 
@@ -867,6 +880,15 @@ and run vm =
           let request = pop vm in
           raise (VmSuspended (request, vm))
 
+        (* ---- Extension dispatch fallthrough ----
+           Opcode partition (see plans/sx-vm-opcode-extension.md):
+             0       reserved / NOP
+             1-199   core opcodes (current ceiling 175 = OP_DEC)
+             200-247 extension opcodes (registered via Sx_vm_extensions)
+             248-255 reserved for future expansion / multi-byte
+           Any opcode ≥ 200 routes through the extension registry. *)
+        | op when op >= 200 -> !extension_dispatch_ref op vm frame
+
         | opcode ->
           raise (Eval_error (Printf.sprintf "VM: unknown opcode %d at ip=%d"
                                opcode (frame.ip - 1)))