Merge #335: Make FutureResult non-Send when no-std

936c724 fix(persisted): Make `FutureResult` non-`Send` when no-`std` (Fedeparma74)

Pull request description:

  ### Description

  This PR updates the `FutureResult` type alias in the async persister implementation to support **non-`Send` futures** when the `std` feature is disabled.

  Previously, the `FutureResult` type always required the `Send` bound, which prevented `no_std` environments from using non-`Send` futures. With this change, the bound is applied only when the `std` feature is enabled.

  This improves compatibility for embedded or `no_std` targets while keeping full `Send` safety for standard environments.

  ### Notes to the reviewers

  The change is minimal and fully backwards compatible.
  It only affects compilation under `no_std` builds and does not modify any runtime behavior or APIs under the `std` feature.

  Please confirm that this approach aligns with the intended async API design and does not conflict with downstream usage expectations.

  ### Changelog notice

  #### Changed

  * Relaxed the `FutureResult` type alias to allow non-`Send` futures in `no_std` builds.

ACKs for top commit:
  ValuedMammal:
    ACK 936c724
  luisschwab:
    ACK 936c724

Tree-SHA512: 388922a023246be786c1d475d02d800ebf69be3759492ee4f20a7be47667a013bbd883742d838bf8479447fd19a0ec9d280d39d9da6c75d15700817f4c3280fe

Author: ValuedMammal

src/wallet/persisted.rs

@@ -55,7 +55,10 @@ pub trait WalletPersister {
     fn persist(persister: &mut Self, changeset: &ChangeSet) -> Result<(), Self::Error>;
 }
 
+#[cfg(feature = "std")]
 type FutureResult<'a, T, E> = Pin<Box<dyn Future<Output = Result<T, E>> + Send + 'a>>;
+#[cfg(not(feature = "std"))]
+type FutureResult<'a, T, E> = Pin<Box<dyn Future<Output = Result<T, E>> + 'a>>;
 
 /// Async trait that persists [`PersistedWallet`].
 ///