fix: use SECONDS_PER_SLOT when deriving slots from time

[unnawut]

🗒️ Description

Most slot calculation were using INTERVALS_PER_SLOT to derive slot number from time. This is only a coincidence that it's correct because both SECONDS_PER_SLOT = 4 and INTERVALS_PER_SLOT = 4.

A quick mental check on this is imagine we have more sub-slots cutoffs but slot time is still the same at 4 seconds, so INTERVALS_PER_SLOT = 6 but SECONDS_PER_SLOT = 4.

Then current_slot = Slot(self.time // INTERVALS_PER_SLOT) becomes current_slot = Slot(self.time // 6) which gives the wrong slot number at 4 seconds per slot. It should be current_slot = Slot(self.time // SECONDS_PER_SLOT)

🔗 Related Issues or PRs

✅ Checklist

• Ran tox checks to avoid unnecessary CI fails:

  uvx tox

• Considered adding appropriate tests for the changes.
• Considered updating the online docs in the ./docs/ directory.

[tcoratger]

Thanks, I thought we had eliminated the bugs with this fix #75 (comment) but apparently there were still some left, I hope it's okay now.