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.
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.
