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>

docs/language/angle/angle-diff_degrees.md

@@ -0,0 +1,19 @@
+---
+id: angle-diff_degrees
+name: Angle.diff_degrees
+category: angle
+kind: namespace-method
+tokens: Angle.diff_degrees
+sig: Angle.diff_degrees(a: int, b: int) -> int
+tip: The signed difference between two headings in whole degrees.
+order: 20
+ns: Angle
+member: diff_degrees
+---
+
+<code>b - a</code> wrapped into -180 .. 180, for integer-degree headings such as <code>TopDown.aim_angle</code> and <code>IVec2.heading</code>.
+
+```ludic
+# doc-check: skip — illustrative
+if abs(Angle.diff_degrees(a: aim, b: IVec2.heading(hero, foe))) <= 50 { hit() }
+```