Optimize computation of unrealized checkpoints

[tushar994]

Fork choice: skip process_justification_and_finalization when result is provably unchanged

Implements the optimization described in the existing TODO(Grandine Team) comment in fork_choice_store/src/store.rs. Referenced lighthouse for the appropriate change.

When computing unrealized checkpoints for a new block in epoch N, if the parent's unrealized justified checkpoint is already at epoch N and its unrealized finalized checkpoint is at epoch N−1, no block in this epoch can push those checkpoints any higher. The child block inherits the parent's values directly and the O(validators) process_justification_and_finalization call is skipped. This fires roughly one third of the time under normal finality conditions.

Tests

cargo test --release --no-default-features --features default-networks,blst \
  --workspace \
  --exclude zkvm_host --exclude zkvm_guest_risc0 \
  --exclude c_grandine --exclude csharp_grandine \
  --exclude slashing_protection --exclude http_api \
  -p fork_choice_control

all tests passed

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