FCR: incorporate fixes and spec ceil expression

[dapplion]

Summary

• incorporate the changes from dapplion/lighthouse:fcr-fixes
• keep the PR FCR: align committee weight estimate with spec #96 spec-shaped committee-weight estimate structure
• replace div_ceil(1000) with the spec-shaped (estimate + 999) // 1000 arithmetic using checked ops
• handle missing checkpoint lookups without a dummy checkpoint sentinel
• make get_equivocation_score iterate the equivocating set directly

Tests

• cargo check -p fast_confirmation
• pre-push make lint hook

Earlier on this branch before the latest cleanup:

• cargo test -p fast_confirmation
• cargo test -p ef_tests --features ef_tests fast_confirmation