vm-ext: phase D — extensions/ subtree + test_ext + opcode_name lookup

lib/extensions/ becomes the new home for VM extensions, wired in via
(include_subdirs unqualified). README documents the registration
pattern, opcode-ID range conventions (200-209 guest_vm, 210-219
inline test, 220-229 test_ext, 230-247 ports), and naming rules.

extensions/test_ext.ml is the canonical worked example — two
operand-less opcodes (220 push 42, 221 double TOS) carrying a per-
extension state slot (TestExtState invocation counter). Test_ext.register
called from run_tests.ml at the start of the Phase D suite, on top of
the inline test_reg from earlier suites (disjoint opcode IDs).

Sx_vm.opcode_name now consults extension_opcode_name_ref (forward ref
in the same style as extension_dispatch_ref), so disassemble shows
extension opcodes by name instead of UNKNOWN_n. Registry maintains
name_of_id_table and installs the lookup at module init.

Tests: 5 new foundation cases — primitive resolves test_ext name,
end-to-end bytecode (push + double + return → 84), disassemble shows
"test_ext.OP_TEST_PUSH_42" / "test_ext.OP_TEST_DOUBLE_TOS",
unregistered ext opcodes still fall back to UNKNOWN_n, invocation
counter records the two dispatches. +5 pass vs Phase C baseline, no
regressions across 11 conformance suites.

hosts/ocaml/lib/sx_vm_extensions.ml

@@ -20,6 +20,10 @@ let by_id : (int, handler) Hashtbl.t = Hashtbl.create 64
 (** opcode_name → opcode_id *)
 let by_name : (string, int) Hashtbl.t = Hashtbl.create 64
 
+(** opcode_id → opcode_name (reverse of [by_name]; used by
+    [Sx_vm.opcode_name] for disassembly). *)
+let name_of_id_table : (int, string) Hashtbl.t = Hashtbl.create 64
+
 (** extension_name → state *)
 let states : (string, extension_state) Hashtbl.t = Hashtbl.create 8
 
@@ -58,7 +62,8 @@ let register (m : (module EXTENSION)) =
   Hashtbl.add states M.name st;
   List.iter (fun (id, opname, h) ->
     Hashtbl.add by_id id h;
-    Hashtbl.add by_name opname id
+    Hashtbl.add by_name opname id;
+    Hashtbl.add name_of_id_table id opname
   ) ops;
   extensions := M.name :: !extensions
 
@@ -66,6 +71,10 @@ let register (m : (module EXTENSION)) =
     extension provides that opcode. *)
 let id_of_name name = Hashtbl.find_opt by_name name
 
+(** Look up the opcode_name for an opcode_id. Returns [None] if no
+    extension provides that opcode. Used by disassembly. *)
+let name_of_id id = Hashtbl.find_opt name_of_id_table id
+
 (** Look up the state of an extension by name. Returns [None] if the
     extension is not registered. *)
 let state_of_extension name = Hashtbl.find_opt states name
@@ -78,13 +87,16 @@ let registered_extensions () = !extensions
 let _reset_for_tests () =
   Hashtbl.clear by_id;
   Hashtbl.clear by_name;
+  Hashtbl.clear name_of_id_table;
   Hashtbl.clear states;
   extensions := []
 
-(** Install our [dispatch] into [Sx_vm.extension_dispatch_ref], replacing
-    the Phase A stub. Idempotent. Called automatically at module init. *)
+(** Install our [dispatch] into [Sx_vm.extension_dispatch_ref] and our
+    [name_of_id] into [Sx_vm.extension_opcode_name_ref], replacing
+    the Phase A stubs. Idempotent. Called automatically at module init. *)
 let install_dispatch () =
-  Sx_vm.extension_dispatch_ref := dispatch
+  Sx_vm.extension_dispatch_ref := dispatch;
+  Sx_vm.extension_opcode_name_ref := name_of_id
 
 let () = install_dispatch ()