feat(ecs): engine-owned systems auto-tick user components (#43)

Adds the ECS hook issues #43 and #47 named as their real dependency: a
system the *engine* owns, inserted into the frame loop over a component a
game merely declares and carries — no `handler` wired.

- runtime/native/systems.ludic: esys_spriteanim (SpriteAnim frame advance:
  loop/once/pingpong) and esys_motion (Motion value tween: linear/in/out/
  in-out), both on the by-name reflection ABI, integer + deterministic.
- backend: emit_engine_systems_for_phase inserts the calls after every user
  handler in a phase (auto-loop and the drivable tick helpers alike);
  uses_engine_systems() drives the systems.ludic splice, the world-table
  force-emit, and makes a component-only game count as a systems game.
- A game that declares neither component is byte-for-byte unchanged.

Worked example + regression: examples/library/anim_ecs.ludic. Full suite
73 passed, self-host C-free bootstrap fixpoint intact.

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

selfhost/backend/game/emit_ecs.ludic

@@ -8,6 +8,10 @@ const MAX_ENT: int = 1024
 function has_systems() -> bool {
   var i = 0
   while i < len(prog) { if prog[i].kind == N_SYS { return true };  i = i + 1 }
+  # a game with no hand-written handler but a well-known engine component still
+  # runs a frame loop — the engine owns the system that ticks that component
+  # (#43/#47). Treat it as a systems game so the loop / tick helpers are emitted.
+  if uses_engine_systems() { return true }
   return false
 }
 function has_models() -> bool {
@@ -15,6 +19,19 @@ function has_models() -> bool {
   while i < len(prog) { if prog[i].kind == N_ARCH { return true };  i = i + 1 }
   return false
 }
+# does the program declare a well-known engine component? Each one is auto-ticked
+# by an engine-owned system (systems.ludic, wired in emit_engine_systems_for_
+# phase). Answering this drives the systems.ludic splice (parse.ludic) and the
+# reflection-ABI force-emit (emit_decl) — a game with none is byte-for-byte the
+# same as before the feature existed. Keep this list in sync with the phase table
+# in emit_engine_systems_for_phase.
+function uses_engine_systems() -> bool {
+  if find_comp("SpriteAnim") != null { return true }
+  if find_comp("Motion") != null { return true }
+  if find_comp("Light2D") != null { return true }
+  if find_comp("Occluder") != null { return true }
+  return false
+}
 # N5: does the program have an `entry` block? A game with both handlers and an
 # `entry` drives its own loop (calling tick_fixed/tick_render), instead of the
 # compiler's auto frame loop. A game with handlers and no entry uses the auto-loop.