feat(engine): #90 atlas-aware Sprite component, #91 become from listeners, 0.3.x ergonomics batch

Closes the two open issues and lands the pending unreleased batch:

- #90: `Sprite { atlas: 1 }` routes esys_sprite through atlas_draw_ex
  (scale/flip/tint), so cell / cell_span / strip ids of any size draw
  through the engine sprite-render system. examples/library/sprite_atlas
  is the pixel-readback regression.
- #91: `become` from an @On(Event) listener / global handler / plain
  function no longer segfaults the compiler; it emits @L_scene_leave()
  (a dispatch on the live scene id) so the leaving scene's on-exit runs.
  UI_* handles are readable from any code (widget table built on first
  use). examples/library/scene_menus covers it.
- fix: a windowed `ludicc -o` build that reaches the audio runtime only
  through the atlas/Assets preload import now links audio.ll +
  AVFoundation (the audio backend link was gated on a game-level
  Audio.* call, so any windowed game declaring Sprite failed to link).
- the hand-written "Unreleased" CHANGELOG section is converted to
  changesets under changes/ so `x release` generates it.
- plus the batch: engine-driven retained UI + UiClicked event, Overlay
  phase, TileSkin tilemap-render system, Key.* constants, Font/Ui/File
  namespaces, Sprite.strip, prefabs, managers, countdown fields,
  enum-typed machines, layer @Queries, ludic.prefs / ludic.dungeon
  packages, Ai.seek pathing, Solids.solid2, cursor confine (mode 3)
  fix, shooter centre-aim fix, reserved-word function diagnostic.

Verified: x test (124/124), x test-tools, check-impl, check-vocabulary,
check-docs, docs-gen + docs-check, bootstrap-cfree (seed is a fixpoint).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

selfhost/backend/emit_decl.ludic

@@ -13,6 +13,7 @@ function emit_params_sig(d: Node) -> void {
 
 function emit_fn(d: Node) -> void {
   ll_t = 0;  ll_lbl = 0;  g_term = false;  loc_reset();  nloop = 0
+  g_cur_scene = null                            # a function belongs to no scene: `become` leaves the live one
   ret_ty = d.ty
   let fbody = buf_new()
   falloc = buf_new()
@@ -57,6 +58,7 @@ function emit_main(d: Node) -> void {
   # its framebuffer allocated. Headless it only allocates — no window, no output —
   # so a non-rendering entry game is unchanged.
   if (find_fn("rt_init") != null) { emit("  call void @fn_rt_init()\n") }
+  emit("  call void @L_init_globals()\n")
   emit_block(d.a)
   if not g_term { emit("  br label %ret\n") }
   emit("ret:\n")
@@ -106,6 +108,7 @@ function emit_test_runner() -> void {
   emit("  store ptr %argv, ptr @L_argv\n")
   emit("  %failed = alloca i32\n  store i32 0, ptr %failed\n")
   if has_ecs() { emit("  call void @rt_init()\n") }
+  emit("  call void @L_init_globals()\n")
   i = 0
   while i < len(g_tests) {
     let okmsg = emit_str_const(`ok   - {g_tests[i].s}`)
@@ -166,6 +169,7 @@ function emit_program() -> void {
     }
     i = i + 1
   }
+  emit_global_init_fn()                           # @L_init_globals: the non-constant `var` initializers
   if len(g_events) > 0 { emit_event_fns() }       # EV0: @ev_<E> event-dispatch functions
   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() }