vm-ext: phase C — extension-opcode-id SX primitive

Registers extension-opcode-id from sx_vm_extensions.ml module init.
Lives downstream of both sx_primitives and sx_vm to avoid a build
cycle. Accepts a string or symbol; returns Integer id when the opcode
is registered, Nil otherwise.

Compilers (lib/compiler.sx) call this to emit extension opcodes by
name. Returning Nil rather than failing on unknown names lets a port's
optimization opt in per-build — missing extensions degrade to slower
correct execution.

Tests: 5 new foundation cases — registered lookup, unknown → nil,
symbol arg, zero-arg + integer-arg rejection. +5 pass vs Phase B
baseline, no regressions across 11 conformance suites.

plans/sx-vm-opcode-extension.md

@@ -279,8 +279,8 @@ compiler in `lib/compiler.sx` can call it to emit extension opcodes by name.
 Does not require any extension to actually exist — the primitive returns
 `nil` for unknown names, and the compiler falls back.
 
-- [ ] Register `extension-opcode-id` in `sx_primitives.ml`.
-- [ ] Returns `Integer id` when registered, `Nil` otherwise.
+- [x] Register `extension-opcode-id` in `sx_primitives.ml`.
+- [x] Returns `Integer id` when registered, `Nil` otherwise.
 
 **Tests:**
 - Primitive returns nil for unknown name.
@@ -457,6 +457,20 @@ familiarity.
 
 Newest first.
 
+- **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).
+  Accepts a string or symbol; returns `Integer id` for registered
+  opcode names, `Nil` for unknown — so a missing extension at compile
+  time degrades to a fallback rather than failure. 5 new foundation
+  tests (`extension-opcode-id primitive` suite): registered lookup,
+  unknown → nil, symbol arg, zero-arg rejection, integer-arg
+  rejection. +5 pass vs Phase B baseline (4821 vs 4816), 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 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`,