refactor: extract flashblock timing code and test it

[akundaz]

📝 Summary

Extracts flashblock timing logic from payload.rs into a dedicated timing.rs module. Consolidates duplicate timing calculation code paths into pure functions with clear branching.

Also precomputes the times at which we trigger flashblock builds. This allows for thorough unit testing and decouples the trigger loop from the timing calculations. So the tests don't need any kind of mocking or interaction with tokio.

In addition to all the unit tests for computing flashblock send times I also manually tested locally using builder playground.

This also deprecates the following flags

• --flashblocks.fixed -- we only support dynamics timing calculations
• --flashblocks.build-at-interval-end -- always choose to build at the end
• --flashblocks.leeway-time -- only was used when building at the start

💡 Motivation and Context

The flashblock timing code is currently tangled with less related code in payload.rs. There are also separate code paths for build_at_interval_end and fixed/dynamic mode, leading to duplicated logic. This PR addresses these quality concerns.

As we're working on making flashblocks more reliable, we need to have code clear, isolated, and testable enough so we can make changes quickly.

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✅ I have completed the following steps:

• Run make lint
• Run make test
• Added tests (if applicable)

[avalonche]

we can remove --flashblocks.build-at-interval-end --flashblocks.leeway-time and --flashblocks.fixed flags to simplify the logic

[julio4]

What exactly changed that made the total flashblocks jump from 34 to 42?

[akundaz]

This old test used leeway_time to calculate the deadline and I think I ported the calculation using that field slightly incorrectly.

With the deprecation of leeway_time though I added the equivalent test using the end buffer: progressive_lag_reduces_flashblocks. It also has assertions for the number of flashblocks for each block so we'll be able to catch this kind of drift more precisely in the future.

[akundaz]

we can remove --flashblocks.build-at-interval-end --flashblocks.leeway-time and --flashblocks.fixed flags to simplify the logic

Done

[avalonche]

could we keep this visualisation in the code or docs? I think its easier to understand when visualised like this