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.

plans/sx-vm-opcode-extension.md

@@ -298,10 +298,10 @@ Compiler test: write SX that triggers the test compiler-extension to emit
 `OP_TEST_PUSH_42`, then verify the VM executes it correctly via
 `bytecode-inspect` and `vm-trace`.
 
-- [ ] `test_ext.ml` registers two opcodes.
-- [ ] Wired into the build (extensions registered at startup).
-- [ ] Bytecode emission via name lookup produces the right ID.
-- [ ] `bytecode-inspect` shows the opcode by name.
+- [x] `test_ext.ml` registers two opcodes.
+- [x] Wired into the build (extensions registered at startup).
+- [x] Bytecode emission via name lookup produces the right ID.
+- [x] `bytecode-inspect` shows the opcode by name.
 
 **Tests:**
 - Bytecode emission via name lookup produces the right ID.
@@ -457,6 +457,29 @@ familiarity.
 
 Newest first.
 
+- **2026-05-15** — Phase D done. New `hosts/ocaml/lib/extensions/` subtree
+  wired into the `sx` library via `(include_subdirs unqualified)`.
+  `extensions/test_ext.ml` is the canonical worked example: two
+  operand-less opcodes (`test_ext.OP_TEST_PUSH_42` = 220,
+  `test_ext.OP_TEST_DOUBLE_TOS` = 221) carrying `TestExtState` (an
+  invocation counter that exercises the per-extension state slot).
+  `extensions/README.md` documents the registration pattern, opcode-ID
+  range conventions, and naming rules.
+
+  `Sx_vm.opcode_name` now consults `extension_opcode_name_ref` (forward
+  ref) so disassembly shows extension opcodes by name instead of
+  `UNKNOWN_n`. Registry maintains `name_of_id_table` (reverse of
+  `by_name`) and installs the lookup at module init alongside the
+  dispatch ref. 5 new foundation tests (`extensions/test_ext` suite):
+  `extension-opcode-id` finds OP_TEST_PUSH_42, end-to-end bytecode runs
+  to 84, disassemble shows opcode names, unregistered ext opcodes still
+  fall back to UNKNOWN_n, per-extension state counter increments.
+  +5 pass vs Phase C baseline (4826 vs 4821), 1111 pre-existing failures
+  unchanged. Conformance suites green: erlang 530/530, haskell 285/285,
+  datalog 276/276, prolog 590/590, smalltalk 847/847, common-lisp
+  487/487, apl 562/562, js 148/148, forth 632/638 (pre-existing), tcl
+  3/4 (pre-existing), ocaml-on-sx unit 607/607.
+
 - **2026-05-15** — Phase C done. `extension-opcode-id` SX primitive
   registered from `sx_vm_extensions.ml` module init (avoids the
   `sx_primitives ↔ sx_vm` cycle by registering downstream of both).