Serve verified uncommitted blocks to seq-only lookups in CachedStorage

[samliok]

Problem

CachedStorage keeps verified-but-not-yet-finalized blocks in a cache keyed by digest. The msm sealing-block checks (buildBlockOrTransitionEpoch, areWeReadyToTransitionEpoch) look blocks up by sequence only and pass a zero digest, so they can never hit the cache. The lookup falls through to committed storage, and while the sealing block is notarized but not yet indexed, block building fails with failed to retrieve sealing block for previous epoch. The failed build is not retried, so the leader misses its round and the round empty-notarizes.

The window is the normal pipelining case right after a sealing block: the next leader builds on the notarized sealing block before it finalizes. Surfaced as a deterministic-ish hang in a new multi-node instance test where round 2's build races round 1's commit (test lands separately with the instance test harness).

Fix

When a lookup misses the cache and the digest is zero, scan the cache by sequence before falling back to storage. A hit returns the block with a nil finalization, matching the documented cache invariant (cached means not yet finalized), so callers correctly compute isSealingBlockFinalized = false instead of erroring. Digest-based lookups are unchanged: a real digest that is not cached still resolves through committed storage.

Related: #495 and #496 fix the error masking that hid this failure (the msm error was dropped and misreported as a context cancellation).

🤖 Generated with Claude Code

[yacovm]

Let's add a test that tests the added lines.