Cherry-pick fixes from okx/xlayer-reth#169 into op-rbuilder:

- Scheduler now sends immediate first flashblock trigger on FCU instead
  of waiting idle for the first interval offset
- Target flashblocks calculated via div_ceil(remaining_time, interval)
  instead of block_time / interval, aligning target count with send times
- Late FCU backwards compatibility: returns block_time instead of zero
  duration so payload building still proceeds normally
- Send offset correctly applied to every flashblock interval, not just
  first and last
- Simplified interval computation: removed truncation and deadline append
- Guard check prevents building flashblocks past target count
- Early return when target_flashblocks is zero
- Added 12 comprehensive async unit tests with tokio test-util

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Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>