feat(blockchain): count proposals whose build tick was skipped

[zexoverz]

🗒️ Description / Motivation

When a late tick jumps over an interval 4 where we propose the next slot, that block is never built and lean_block_building_failures_total stays at 0. This makes the case visible.

What Changed

• crates/blockchain/src/lib.rs: skipped_build_slots finds the interval-4 ticks a late tick jumped over; on_tick logs a warn and bumps the counter for each one where we are the proposer.
• crates/blockchain/src/metrics.rs: lean_block_proposals_missed_total.
• docs/metrics.md: one row for the new metric.

Correctness / Behavior Guarantees

Log and metric only. The root cause at the rotation wrap is left alone.

Tests Added / Run

Three unit tests for skipped_build_slots. They cover the helper, not its call inside on_tick.

Related Issues / PRs

• Related to Proposer silently skips its slot after every proposer-rotation wrap (slot ≡ 1 mod 512); no failure counted #602

✅ Verification Checklist

• Ran make fmt — clean
• Ran make lint (clippy with -D warnings) — clean
• Ran make test (cargo test --workspace --profile release-fast) — all passing