feat: converter reads the Phone library's register statement, named stores and generators

`init phone register` becomes a default (the group chat) plus a function of library calls that runs with
the other defaults of a new game (phone_register.py). `init/python [early|hide] in some.store` wraps its
code in an in_store node and `default some.store.name` may name a store; yield works as an expression
(generator functions are marked gen); `import x as y`, `transform -150 name(...)`, `init:` with if/else,
`config.special_namespaces` lines, `at` lists and tuples, transforms made by Python functions, `choice`
weights that are expressions, `__len__`/`__bool__`, and the whitelist for os, calendar, itertools,
pygame_sdl2 and the new runtime. The converter also records the size of every picture it converts
(`imageSizes`) so `renpy.image_size` answers without loading.

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

backend/app/converter/atl.py

@@ -399,7 +399,9 @@ class AtlParser:
                 break
             weight: Any = 1.0
             if first == "choice" and len(words) > 1:
-                weight = self.number(words[1], current) or 1.0
+                # the weight may be any expression, `not flag` included; a false flag means no chance at all
+                weight = self.number(" ".join(words[1:]), current)
+                weight = 1.0 if weight is None else weight
             group.append((weight, self.block(current.block)))
             j += 1
         if first == "parallel":