vm-ext: phase B — extension registry module

sx_vm_extension.ml: handler type, extensible extension_state variant,
EXTENSION first-class module signature.

sx_vm_extensions.ml: register / dispatch / id_of_name /
state_of_extension. install_dispatch () runs at module init,
swapping Phase A's stub for the real registry. Rejects out-of-range
opcode IDs (must be 200-247), duplicate IDs, duplicate names, and
duplicate extension names.

Tests: 9 new foundation cases — lookup hits/misses, end-to-end VM
dispatch including opcode composition, all four rejection paths.
+9 pass vs Phase A baseline, no regressions across 11 conformance
suites.

plans/sx-vm-opcode-extension.md

@@ -258,10 +258,10 @@ Five sub-phases in dependency order. Each is testable in isolation.
 opcodes yet. Phase B's module init installs the real `dispatch` into
 `Sx_vm.extension_dispatch_ref`, replacing Phase A's stub.
 
-- [ ] `Sx_vm_extension` interface module (handler type, EXTENSION sig).
-- [ ] `Sx_vm_extensions` registry module (`register`, `dispatch`,
+- [x] `Sx_vm_extension` interface module (handler type, EXTENSION sig).
+- [x] `Sx_vm_extensions` registry module (`register`, `dispatch`,
       `id_of_name`, `state_of_extension`).
-- [ ] Wire the registry's `dispatch` into `Sx_vm.extension_dispatch_ref` at
+- [x] Wire the registry's `dispatch` into `Sx_vm.extension_dispatch_ref` at
       module init.
 
 **Tests:**
@@ -457,6 +457,24 @@ familiarity.
 
 Newest first.
 
+- **2026-05-14** — Phase B done. Added `hosts/ocaml/lib/sx_vm_extension.ml`
+  (interface: `handler` type, `extension_state` extensible variant,
+  `EXTENSION` module type) and `sx_vm_extensions.ml` (registry: `register`,
+  `dispatch`, `id_of_name`, `state_of_extension`, `_reset_for_tests`).
+  `let () = install_dispatch ()` at module init replaces Phase A's stub
+  with the real registry dispatch — Phase A behavior preserved (empty
+  registry still raises `Invalid_opcode` for unregistered ops). Registry
+  rejects opcode IDs outside 200-247, duplicate IDs, duplicate names, and
+  duplicate extension names. 9 new foundation tests (`vm-extension-registry`
+  suite): id_of_name resolve+miss, state_of_extension resolve+miss,
+  end-to-end VM dispatch (push 42), opcode composition (push 42 → double
+  → 84), duplicate-id / out-of-range / duplicate-name rejection. +9 pass
+  vs Phase A baseline (4816 vs 4807), 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.
+
 - **2026-05-14** — Phase A done. Added `Invalid_opcode of int` exception,
   `extension_dispatch_ref` (default raises `Invalid_opcode op`), and the
   `| op when op >= 200 -> !extension_dispatch_ref op vm frame` arm before the