Duration is reflected as Opaque instead of Struct

## Bevy version

0.15.0

## What you did

I was trying to use Blenvy to serialize Duration types, but they were just empty strings instead of the `secs: u64, nanos: u64` that I expected.

Turns out that in bevy_reflect, there's this little snippet in [the reflect std impls](https://github.com/bevyengine/bevy/blob/65835f535493a14d4fabe3c1569de61be2e884b1/crates/bevy_reflect/src/impls/std.rs#L119) (`impl_reflect_opaque!(::bevy_utils::Duration`) that impls it as Opaque instead of Struct, rendering it unserializable to Blenvy since it can't access the internal struct members.

Start in crates/bevy_reflect/src/impls/std.rs around the impl_reflect_opaque!(::bevy_utils::Duration) entry. Check the existing reflection implementations and relevant serialization coverage, then verify that Duration is reflected as a Struct exposing secs and nanos rather than as an Opaque value.

Written by the indexing model from the issue text.