Fix leak in manganis iOS FFI wrappers: hold Retained<AnyObject> instead of a raw pointer

[nicoburns]

Summary

Fixes #5722.

The iOS/macOS wrappers generated by #[manganis::ffi] constructed their Objective-C instance with alloc + init (+1 retain) and stored it as a bare *mut AnyObject, so dropping the Rust value never sent release — every Bridge::new() leaked its object (and any system resource it owned, e.g. a CLLocationManager). The Android arm already had correct ownership via jni::objects::GlobalRef.

The generated type now holds objc2::rc::Retained<AnyObject>, which takes over the +1 reference and releases on drop:

pub struct #name { inner: Retained<AnyObject> }

// new():
let instance: *mut AnyObject = msg_send![class, alloc];
let instance: *mut AnyObject = msg_send![instance, init];
let inner = Retained::from_raw(instance).ok_or("Failed to initialize instance")?; // replaces the null check

// method calls on a receiver:
msg_send![Retained::as_ptr(&this.inner) as *mut AnyObject, selector...]

from_raw(ptr) likewise wraps the pointer in Retained::from_raw and is documented as taking ownership of a +1 reference (it now panics on null).

Verified the generated code shape type-checks against objc2 0.6.4 on aarch64-apple-darwin (full cross-check of the macro output isn't possible on Linux CI hosts since objc2-exception-helper needs an ObjC compiler).

Link to Devin session: https://dioxus.staging.devinenterprise.com/sessions/cd9de105cf904808b16b468c829b5ac1
Requested by: @nicoburns

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