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/bin/run_tests.ml

@@ -1518,7 +1518,86 @@ let run_foundation_tests () =
      Printf.printf "  FAIL: integer arg should have raised\n"
    with Sx_types.Eval_error _ ->
      incr pass_count;
-     Printf.printf "  PASS: integer arg rejected\n")
+     Printf.printf "  PASS: integer arg rejected\n");
+
+  Printf.printf "\nSuite: extensions/test_ext (canonical extension)\n";
+  (* Phase D: the real test extension lives at lib/extensions/test_ext.ml.
+     Register it on top of the inline test_reg from earlier suites — the
+     two use disjoint opcode IDs (210/211 vs 220/221) so they coexist. *)
+  Test_ext.register ();
+
+  (* Lookup via the public primitive should now find OP_TEST_PUSH_42. *)
+  (match prim [String "test_ext.OP_TEST_PUSH_42"] with
+   | Integer 220 ->
+     incr pass_count;
+     Printf.printf "  PASS: extension-opcode-id finds test_ext.OP_TEST_PUSH_42\n"
+   | other ->
+     incr fail_count;
+     Printf.printf "  FAIL: opcode lookup: got %s\n" (Sx_types.inspect other));
+
+  (* End-to-end: PUSH_42 + DOUBLE_TOS + RETURN. *)
+  (let globals = Hashtbl.create 1 in
+   try
+     match Sx_vm.execute_module (make_bc_seq [| 220; 221; 50 |]) globals with
+     | Integer 84 ->
+       incr pass_count;
+       Printf.printf "  PASS: extensions/test_ext bytecode executes (84)\n"
+     | other ->
+       incr fail_count;
+       Printf.printf "  FAIL: test_ext bytecode result: got %s\n"
+         (Sx_types.inspect other)
+   with exn ->
+     incr fail_count;
+     Printf.printf "  FAIL: test_ext bytecode raised: %s\n"
+       (Printexc.to_string exn));
+
+  (* Disassembly: opcode_name should resolve 220/221 via the registry,
+     not fall back to UNKNOWN_220 / UNKNOWN_221.  disassemble returns a
+     Dict; the instruction list lives at key "bytecode". *)
+  (let code = make_bc_seq [| 220; 221; 50 |] in
+   let dis = Sx_vm.disassemble code in
+   let entries = match dis with
+     | Dict d -> (match Hashtbl.find_opt d "bytecode" with
+                  | Some (List es) -> es
+                  | _ -> [])
+     | _ -> []
+   in
+   let names = List.filter_map (fun entry -> match entry with
+     | Dict d ->
+       (match Hashtbl.find_opt d "opcode" with
+        | Some (String name) -> Some name
+        | _ -> None)
+     | _ -> None) entries
+   in
+   let has name = List.mem name names in
+   if has "test_ext.OP_TEST_PUSH_42" && has "test_ext.OP_TEST_DOUBLE_TOS" then begin
+     incr pass_count;
+     Printf.printf "  PASS: disassemble shows extension opcode names\n"
+   end else begin
+     incr fail_count;
+     Printf.printf "  FAIL: disassemble names: [%s]\n" (String.concat ", " names)
+   end);
+
+  (* Sanity: opcode_name on an unregistered extension opcode still
+     returns UNKNOWN_n.  Pick 230 — out of test_ext's range. *)
+  (match Sx_vm.opcode_name 230 with
+   | "UNKNOWN_230" ->
+     incr pass_count;
+     Printf.printf "  PASS: unregistered ext opcode falls back to UNKNOWN_n\n"
+   | other ->
+     incr fail_count;
+     Printf.printf "  FAIL: opcode_name 230: got %s\n" other);
+
+  (* Per-extension state: invocation_count should reflect the two opcodes
+     that ran in the dispatch test above. *)
+  (match Test_ext.invocation_count () with
+   | Some n when n >= 2 ->
+     incr pass_count;
+     Printf.printf "  PASS: extension state recorded %d invocations\n" n
+   | other ->
+     incr fail_count;
+     Printf.printf "  FAIL: invocation_count: %s\n"
+       (match other with Some n -> string_of_int n | None -> "None"))
 
 
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