🤖 Cover epoch minus two in FCR assignments

[dapplion]

Links audit/review artifact: #100

Implements FCR-05 from the audit:

• Extends SlotAssignments coverage to include current_epoch - 2 for reconfirmation empty-slot discount paths.
• Uses a detached historical committee cache for the extra epoch without mutating the normal previous/current/next state caches.
• Adds a regression test for epoch-minus-two range coverage.

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