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.
giles
58d7445559