Instant fallback fix for aarch64

[chocolacula]

Objective

The current Instant fallback implementation in bevy_platform contains a bug on ARM CPUs, such as Apple Silicon. It appears only under specific conditions — when the std feature is disabled for desktop builds on ARM. For web builds, as I can see, the time implementation differs and does not exhibit this issue. Still, this remains a bug.

In the current implementation, the code reads the system counter value (tick count) and incorrectly interprets it as nanoseconds. We have to additionally divide number by frequency to obtain elapsed time.

Solution

Query the system counter frequency and compute nanoseconds using the correct formula:

// number of second in nanos resolution
 (ticks / freq) * 1_000_000_000 +         // integer part
((ticks % freq) * 1_000_000_000) / freq   // fractional part

Testing

A test was added, but verifying this fix is tricky since it’s platform-specific.
The test checks the correct condition only on aarch64 when the std feature is disabled — which is somewhat cumbersome to configure in bevy’s root project. The best option I found for development is disable crate::cfg::std branch in bevy_platform/src/time/mod.rs

For a visual sanity check, you can run the move_sprite example, if the time calculation is incorrect, the sprite’s movement appears roughly 10x slower than expected.

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[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
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[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-21677

If it's expected, please add the M-Deliberate-Rendering-Change label.

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[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-21677

If it's expected, please add the M-Deliberate-Rendering-Change label.

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[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-21677

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.