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/stdlib/emit_ivec.ludic

@@ -20,6 +20,7 @@ function is_ivec_ns(meth: pointer) -> bool {
   if (meth == "make") or (meth == "zero") or (meth == "x") or (meth == "y") { return true }
   if (meth == "add") or (meth == "sub") or (meth == "scale") or (meth == "dot") { return true }
   if (meth == "equal") or (meth == "manhattan") or (meth == "to_vector") { return true }
+  if (meth == "distance2") or (meth == "within") or (meth == "heading") or (meth == "along") or (meth == "step") { return true }
   return false
 }
 
@@ -39,6 +40,34 @@ function emit_ivec_ns(meth: pointer, e: Node) -> Val {
     let v = emit_expr(e.kids[0])
     return val(vec_y(v.code), "int")
   }
+  if (meth == "distance2") or (meth == "within") {  # squared distance; within(a, b, radius)
+    let a = emit_expr(e.kids[0]);  let b = emit_expr(e.kids[1])
+    let dx = emit_bind(`sub i32 {vec_x(b.code)}, {vec_x(a.code)}`)
+    let dy = emit_bind(`sub i32 {vec_y(b.code)}, {vec_y(a.code)}`)
+    let xx = emit_bind(`mul i32 {dx}, {dx}`)
+    let yy = emit_bind(`mul i32 {dy}, {dy}`)
+    let d2 = emit_bind(`add i32 {xx}, {yy}`)
+    if (meth == "distance2") { return val(d2, "int") }
+    let r = emit_expr(e.kids[2])
+    let rr = emit_bind(`mul i32 {r.code}, {r.code}`)
+    let c = emit_bind(`icmp sle i32 {d2}, {rr}`)
+    return val(emit_bind(`zext i1 {c} to i32`), "bool")
+  }
+  if (meth == "heading") {                          # degrees from a to b (0 = +x, 90 = +y)
+    if find_fn("rt_ivec_heading") == null { perr("IVec2.heading needs the engine runtime") }
+    let a = emit_expr(e.kids[0]);  let b = emit_expr(e.kids[1])
+    return val(emit_bind(`call i32 @fn_rt_ivec_heading(i64 {a.code}, i64 {b.code})`), "int")
+  }
+  if (meth == "along") {                            # the point `distance` along `degrees` from origin
+    if find_fn("rt_ivec_along") == null { perr("IVec2.along needs the engine runtime") }
+    let o = emit_expr(e.kids[0]);  let d = emit_expr(e.kids[1]);  let n = emit_expr(e.kids[2])
+    return val(emit_bind(`call i64 @fn_rt_ivec_along(i64 {o.code}, i32 {d.code}, i32 {n.code})`), "IVec2")
+  }
+  if (meth == "step") {                             # a -1/0/1 unit step along `degrees`
+    if find_fn("rt_ivec_step") == null { perr("IVec2.step needs the engine runtime") }
+    let d = emit_expr(e.kids[0])
+    return val(emit_bind(`call i64 @fn_rt_ivec_step(i32 {d.code})`), "IVec2")
+  }
   if (meth == "add") {                              # component-wise a + b
     let a = emit_expr(e.kids[0]);  let b = emit_expr(e.kids[1])
     let ax = vec_x(a.code);  let ay = vec_y(a.code);  let bx = vec_x(b.code);  let by = vec_y(b.code)