🤖 Fix FCR spec safety edges

[dapplion]

Links audit artifact: #100

Implements the first direct FCR spec/safety slice from the audit:

• FCR-01: use the Gloas bid parent payload hash for FCR safe hash.
• FCR-07: derive SlotAssignments from actual committee ranges instead of inverting floor-division boundaries.
• FCR-08: ignore invalidated latest-message roots and descendants for FCR LMD/FFG support.

Validation run locally:

• cargo fmt --package fast_confirmation --package beacon_chain
• cargo test -p fast_confirmation slot_assignments --lib
• cargo test -p fast_confirmation --lib
• cargo check -p beacon_chain

Intent: reviewable fix branch on top of latest fcr tip.