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/app/converter/rpy_helpers.py

@@ -11,7 +11,7 @@ Node = dict[str, Any]
 
 POSITION_PROPS = {"xpos", "ypos", "xanchor", "yanchor", "xalign", "yalign", "xcenter",
                   "ycenter", "zoom", "xzoom", "yzoom", "alpha", "rotate", "xoffset", "yoffset",
-                  "align", "pos", "anchor", "xysize", "size", "blur"}
+                  "align", "pos", "anchor", "xysize", "size", "blur", "matrixcolor", "xycenter"}
 
 
 _SHOW_KEYWORDS = {"at", "as", "behind", "onlayer", "with", "zorder"}