🤖 Improve FCR lifecycle robustness

[dapplion]

Links audit/review artifact: #100

Implements the lifecycle robustness items from the audit:

• FCR-02: skip FCR during next-slot lookahead recompute.
• FCR-04: refresh head-dependent assignment and balance caches when their dependent root changes, even if the head root is unchanged.
• FCR-06: fall back to finalized when the stored confirmed root is unknown/pruned from proto-array.

Validation run locally:

• cargo fmt --package fast_confirmation --package beacon_chain
• cargo test -p fast_confirmation --lib
• cargo check -p beacon_chain
• git diff --check