Fix RefCell re-entrancy panic during isolate cleanup

[MarcelKaeding]

Summary

• Fixes a Rust panic (RefCell<T> already mutably borrowed) on Android when a Dart isolate exits
• During on_isolate_destroyed, a mutable RefCell borrow is held while retain() drops removed entries — their DropNotifier destructors attempt to re-borrow the same RefCell, causing a panic
• drop_manager.rs: Bind the removed value so it outlives the temporary borrow guard, ensuring the RefCell borrow is released before the value's destructors run
• reader_manager.rs: Collect keys to remove first (immutable borrow), then remove entries one at a time so each mutable borrow is scoped to a single remove() call

Test plan

• cargo check passes with no warnings
• Verify on Android that isolate cleanup no longer panics (reproducer from Rust panic: RefCell already mutably borrowed during isolate cleanup on Android #571)

Fixes #571

🤖 Generated with Claude Code

[knopp]

I'd need to reproduce this first, but bigger issue here is that super_native_extensions and irondash first need to be migrated to native assets before doing another release.