feat: render API shim - a game's renpy.Displayable subclasses are drawn and receive events

Games write their own displayables with render(), renpy.Render, blit, fill, canvas() and renpy.redraw.
render.ts keeps a tree of placed pieces (child renders, ordinary displayables, fills and drawing
commands) and turns it into DOM elements that stay put between redraws; pointer events reach event()
as pygame-style objects, and a returned value ends the interaction. Shaders and GPU properties are
accepted and ignored. The converter whitelists the calls and methods and accepts `import pygame`.

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

frontend/tests/fixtures/displayable.py

@@ -0,0 +1,22 @@
+class Meter(renpy.Displayable):
+    def __init__(self, value, **kwargs):
+        super(Meter, self).__init__(**kwargs)
+        self.value = value
+        self.moves = 0
+
+    def render(self, width, height, st, at):
+        renpy.redraw(self, 0.1)
+        rv = renpy.Render(width, 20)
+        rv.fill("#fff", (0, 0, self.value, 20))
+        canvas = rv.canvas()
+        canvas.circle("#f00", (5, 5), 4)
+        rv.blit(renpy.render("pic.png", width, height, st, at), (3, 4))
+        return rv
+
+    def event(self, ev, x, y, st):
+        if ev.type == pygame.MOUSEMOTION:
+            self.moves += 1
+        raise renpy.IgnoreEvent()
+
+
+meter = Meter(60)