Fix nil pointer dereference when using ctx.allocator in init

[meszmate]

• Defer arena allocator binding until after self is moved to its final location, fixing a dangling pointer when ctx.allocator was used in model.init()
• Reset frame allocator before calling model.init() so ctx.allocator works correctly from the start
• Extract resetFrameAllocator() helper to avoid duplicating arena reset logic
• Add allocator stability test and document allocator lifetimes in README