feat: ATL pictures/contains/function/time, colour matrices and more Python for Ren'Py

ATL
- Image blocks become timelines: picture steps with dissolve, parallel, contains,
  finite repeat, time, function f(trans, st, at), calls of other transforms with
  arguments, on hover/idle events for buttons. Animations stop when their element
  leaves the page
- Pictures defined by an expression (get_face_path("base")) are drawn at show time
- Purely graphical hints (shader, mesh, gl_*, u_*) are accepted and dropped

Colour
- matrixcolor and im.matrix.* as 5x5 matrices combined with `*`, applied through SVG
  filters; im.MatrixColor/Scale/Crop/Blur/Composite and Text/Image as values

Python
- match statement, re/copy/time/datetime/defaultdict/functools.partial, more
  str/dict methods, Character(...) created by code
- narrator("..."), Character calls, renpy.say and renpy.show/hide with expressions in
  Python blocks become script nodes
- Audit follows reachability: functions and classes nothing refers to (custom
  statement lexers, unused tools) no longer block an import

Docs: README and ARCHITECTURE now describe a multi-engine library (GameMaker and
QSP planned). Tests for the new runtime pieces.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

backend/tests/test_compatibility.py

@@ -102,8 +102,7 @@ def test_renpy_atl_animation_becomes_a_timeline_not_a_static_pose():
 
 
 def test_renpy_atl_constructs_without_a_runtime_are_still_rejected():
-    for body in ("function slide", "time 1.0:\n        xalign 1.0", "matrixcolor TintMatrix('#f00')",
-                 "contains:\n        'a.png'"):
+    for body in ("time 1.0:\n        xalign 1.0", "knot (0, 0)", "circles 0.5", "contains 'a.png'"):
         unused = script(f'transform odd:\n    {body}\nlabel start:\n    "Hi"')
         require_supported_scripts(unused)  # nothing shows it, so it cannot break the game
         used = script(f'transform odd:\n    {body}\nlabel start:\n    show eileen at odd\n    "Hi"')