🤖 Preserve FCR vote epochs during V29 migration

[dapplion]

Links audit/review artifact: #100

Implements FCR-09 from the audit:

• Adds a slots-per-epoch-aware V28-to-V29 vote conversion path for schema migration.
• Preserves migrated steady-state latest-message epochs as V29 vote slots so FCR current-target support does not silently read epoch zero.
• Leaves pending next-root votes with only next_slot inferred, because the current root epoch is not represented in V28.

Validation run locally:

• cargo fmt --package proto_array --package beacon_chain
• cargo test -p proto_array v28_vote_conversion --lib
• cargo check -p beacon_chain
• git diff --check

[michaelsproul]

It's too late for this change. We have already deployed v29.