fix(epoch-cache): use TTL-based caching with finalization tracking and correct lag

[spalladino]

Motivation

PR #22153 introduced a hard "finalized block guard" that refuses to compute committees if L1 data isn't finalized. While the safety goal is valid (preventing L1 reorgs from invalidating cached committees), it breaks many tests that don't properly set L1 finalized time and would cause the chain to stall if L1 stops finalizing. This PR takes a different approach that preserves safety while maintaining liveness.

Also fixes the lag parameter: the old code used lagInEpochsForValidatorSet (the looser constraint) instead of lagInEpochsForRandao (the binding one), and computed the sampling timestamp from the slot rather than the epoch start.

Fixes A-680

Approach

Instead of refusing to serve committee data that isn't finalized, use a TTL-based cache: finalized entries are cached permanently, non-finalized entries expire after one Ethereum slot (12s) and get re-fetched from L1. The cache map stores both resolved entries and in-flight promises directly, so concurrent callers for the same epoch coalesce on a single L1 query. On fetch failure, the previous stale entry is restored so the next caller retries cleanly.

Changes

• epoch-cache: Replaced the simple Map<EpochNumber, EpochCommitteeInfo> cache with Map<EpochNumber, CachedEpochEntry | Promise<CachedEpochEntry>>. Each resolved entry carries L1 block provenance metadata (number, hash, timestamp) and a finalized flag. Switched from lagInEpochsForValidatorSet to lagInEpochsForRandao and compute sampling timestamp from epoch start via getStartTimestampForEpoch. Simplified isEscapeHatchOpen to delegate cache management to getCommittee.
• epoch-cache (tests): Updated unit tests for the new cache structure. Added 4 new TTL tests: re-query after TTL, no re-query for finalized, concurrent coalescing, eventual finalization promotion.
• epoch-cache (integration tests): New integration test suite against real Anvil with deployed L1 contracts and 4 validators. Tests finalized committee retrieval, non-finalized TTL refresh, and cache re-fetch after L1 reorg.
• epoch-cache (README): Added comprehensive documentation covering committee computation, LAG values, RANDAO seed, proposer selection, escape hatch, TTL caching with finalization tracking, and configuration.