fix: missed flashblocks at end metric

[akundaz]

The metric and log message at the end of flashblock building was overcounting the number of flashblocks we built by 1 because it was including the index-0 flashblock. So the metric for missed flashblocks would mistakenly show zero because we never miss more than one flashblock at the end.

[Copilot]

Pull request overview

This PR fixes an off-by-one error in the “flashblocks built” accounting used for end-of-build metrics/logging by excluding the index-0 fallback flashblock from the built count, so the “missed flashblocks at end” metric is computed correctly.

Changes:

• Renames progress/stat tracking from flashblock_index (next-to-build) to flashblocks_built (excluding fallback).
• Adds FlashblocksState::scheduled_flashblocks_built() to compute the correct built count via saturating_sub(1).
• Updates metrics and tracing fields to use flashblocks_built for flashblock_count and missing_flashblocks_count.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.