feat: add mod editing, revision merges and secure game storage

Add immutable revisions, PGVN editor, asset replacement, merge and rebase workflows, authenticated media delivery and reachability GC. Harden the script interpreter, pin unrpyc and remove tracked build metadata. Include current compatibility checks, Naninovel import and regression tests.

backend/tests/fixtures/sample/script.rpy

@@ -18,8 +18,8 @@ transform slightleft:
     xalign 0.3 yalign 1.0
 
 init python:
-    def helper():
-        return 1
+
+
     bonus = 5
     persistent.seen_intro = False