feat(compiler): dynamic system registration + --emit-module for binary packages (#64)

The runtime + compiler foundation for prebuilt binary packages, over the
existing reflection C-ABI (EV0–EV8 already gives dynamic components via
ludic_register_prop).

- ludic_register_system(fn, phase) + a registry the frame loop dispatches after
  each phase's own handlers — the systems analogue of ludic_register_prop,
  mirroring the EV6 foreign event-listener array. Emitted for every ECS program;
  a zero-length registry means a non-hosting game is output-identical.
- --emit-module: compile a package to a position-independent module — no main,
  no world table — that declares the host reflection ABI it calls and carries a
  load-time constructor which registers its @System(Phase) functions and runs
  module_init (where it registers its dynamic components). Built as a dylib with
  -undefined dynamic_lookup, it binds ludic_* back to the host image at load.
- @System(Phase) annotation marks a module function as a runtime-registered
  system; the compiler supplies its address (Ludic source cannot take one).
- The reflection ABI (world table) is now emitted for every ECS program, so any
  game can host binary modules with no flag; unused defs dead-strip at -O2, so
  golden renders stay byte-identical and the C-free bootstrap fixpoint holds.

Proven end-to-end: a module dylib registers a component + an Update system; a
host game that never saw its source links it and the system mutates the shared
world each frame. Full suite 87/0, test-tools 30/0, fixpoint intact.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

selfhost/backend/game/emit_world.ludic

@@ -429,6 +429,7 @@ function emit_tick_helpers() -> void {
 # system functions + lifecycle hooks + the drivable tick helpers — shared by the
 # auto-loop game (emit_game_main) and an entry-driven game that owns its own loop.
 function emit_game_defs() -> void {
+  emit_system_registry()                  # #64: @ludic_register_system + the registry globals
   var i = 0
   while i < len(prog) { if prog[i].kind == N_SYS { emit_system_fn(prog[i]) };  i = i + 1 }
   emit_despawn_hooks()