`bevy_reflect` and `bevy_tasks` do not build on platforms without atomics

## Bevy version and features

```toml
bevy = { version = "0.19.0", default-features = false, features = ["default_no_std"] }
```

## Relevant system information

`cargo 1.99.0-nightly (3efb1f477 2026-07-17)`
building for `mipsel-sony-psx`

## Small futures-lite problem

```
error[E0432]: unresolved import `atomic::AtomicUsize`
--> /Users/calin/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/futures-core-0.3.33/src/task/__internal/atomic_waker.rs:5:5
|
5 | use atomic::AtomicUsize;
| ^^^^^^^^-----------
| |
| no `AtomicUsize` in `sync::atomic`
```

As far as I can tell this happens because `bevy_tasks` unconditionally depends on `futures-lite` which depends on `futures-core` *without* the `portable-atomic` feature and no way to enable it (and for reference because this tripped me up, `portable-atomic` is an implicit feature for the optional dependency, it is not listed explicitly but it does exist)
```sh
cargo tree -i futures-core
futures-core v0.3.33
└── futures-lite v2.6.1
└── bevy_tasks v0.19.0
├── bevy_app v0.19.0
...
```

The workaround is to add
```toml
futures-core = { version = "*", default-features = false, features = ["portable-atomic"] }
```
to the project

## `bevy_reflect` uses core::sync liberally

Next, more importantly, `bevy-reflect` does not compile and despite having a `critical-section` feature as it contains code like:
```rs
impl_reflect_for_atomic!(
::core::sync::atomic::AtomicIsize,
::core::sync::atomic::Ordering::SeqCst
);
```
```sh
error[E0425]: cannot find type `AtomicIsize` in module `::core::sync::atomic`
--> /Users/calin/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/bevy_reflect-0.19.0/src/impls/core/sync.rs:135:27
|
135 | ::core::sync::atomic::AtomicIsize,
| ^^^^^^^^^^^ not found in `::core::sync::atomic`
|
help: consider importing this struct
|
1 + use bevy_platform::sync::atomic::AtomicIsize;
|
help: if you import `AtomicIsize`, refer to it directly
|
135 - ::core::sync::atomic::AtomicIsize,
135 + AtomicIsize,
|
```
perhaps it should indeed implement the traits on the `bevy_platform` alternative?

Start by tracing bevy_tasks' futures-lite and futures-core feature configuration, then inspect bevy_reflect/src/impls/core/sync.rs and its use of core::sync::atomic. Reproduce the default_no_std build for mipsel-sony-psx and determine the platform-compatible atomic types and feature wiring; done means the affected crates compile without atomics while existing supported builds remain valid.

Written by the indexing model from the issue text.