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/emit_decl.ludic

@@ -166,7 +166,7 @@ function emit_program() -> void {
     i = i + 1
   }
   if len(g_events) > 0 { emit_event_fns() }       # EV0: @ev_<E> event-dispatch functions
-  if has_ecs() and (len(g_events) > 0 or g_uses_query or g_uses_reflect or g_uses_esys) { emit_world_table() }   # EV2/EV8: the mod reflection ABI (also powers Query.* / Reflect.* / engine systems)
+  if has_ecs() { emit_world_table() }   # EV2/EV8: the mod reflection ABI (powers Query.* / Reflect.* / engine systems, and lets a binary module (#64) link against a shared world). Unused defs dead-strip at -O2, so a game that touches none is output-identical.
   if has_ecs() { emit_ecs_allocator();  emit_snapshot() }
   if has_ecs() { emit_net() }                     # N2/N3: @Sync serializers + @Owned storage (gated internally)
   if has_ui() { emit_ui_build() }
@@ -208,6 +208,7 @@ function emit_program() -> void {
   if g_uses_result { emith("%Result = type { i32, i32, ptr }\n") }   # issue #46: ok/err/try value
   if g_uses_option { emith("%Option = type { i32, i32 }\n") }         # issue #53: some/none value
   emit_cov_runtime()                              # issue #45: --coverage tables + exit dump
+  if g_emit_module { emit_module_glue() }         # issue #64: binary-module host-ABI declares + load-time registration ctor
 }
 
 # issue #45: the line-coverage runtime. Emits the static line table, a parallel