Attestation data can use a stale cached head on empty slots

### Describe the bug

On empty slots, Lodestar can return an outdated head when a validator asks for attestation data.

At the start of each slot, [`updateTime()`](https://github.com/ChainSafe/lodestar/blob/b0ce010779032ffafdf7fc24c4160c37c4314470/packages/fork-choice/src/forkChoice/forkChoice.ts#L1129-L1150) applies queued votes from previous slots. But it only recalculates the head if checkpoints change or fast confirmation runs. Otherwise, the votes are recorded but the cached head stays unchanged.

A [block import](https://github.com/ChainSafe/lodestar/blob/b0ce010779032ffafdf7fc24c4160c37c4314470/packages/beacon-node/src/chain/blocks/importBlock.ts#L300-L301) normally triggers another calculation. If no block is imported and no other head calculation runs, the cached head can remain unchanged until [`PrepareNextSlotScheduler`](https://github.com/ChainSafe/lodestar/blob/b0ce010779032ffafdf7fc24c4160c37c4314470/packages/beacon-node/src/chain/prepareNextSlot.ts#L102-L107) runs at roughly 8 seconds into a 12-second slot.

The attestation deadline is earlier, at roughly 4 seconds. [`produceAttestationData()`](https://github.com/ChainSafe/lodestar/blob/b0ce010779032ffafdf7fc24c4160c37c4314470/packages/beacon-node/src/api/impl/validator/index.ts#L1311-L1314) reads the cached head without refreshing it. A validator can therefore vote for a head that a fresh fork-choice calculation would no longer select.

### Example scenario

Suppose two competing branches, A and B, are already known. Fast confirmation is off, and there is no
checkpoint update or other head calculation before the attestation request.

| Time | Event | Cached head |
| ------------- | ----------------------------------------------------------------- | ------------- |
| Slot N-1, ~8s | Head calculation chooses A. | A |
| Slot N, start | Previous-slot votes are applied. They make B the stronger branch. | A |
| Slot N, ~4s | No block has arrived. A validator asks for attestation data. | A is returned |
| Slot N, ~8s | Scheduled head calculation picks up the changed votes. | B |

Only votes from past slots count here. Votes for slot N
[stay queued until N+1](https://github.com/ChainSafe/lodestar/blob/b0ce010779032ffafdf7fc24c4160c37c4314470/packages/fork-choice/src/forkChoice/forkChoice.ts#L1045-L1065).

If the winning branch does not change, the cached answer is still correct. The concern is when votes
shift support to a competing branch. This report is based on code inspection; the impact on mainnet
has not been measured.

### Expected behavior

Attestation data should use a head that reflects eligible votes already processed by fork choice.
This should also hold when no block arrives in the current slot.

Start with updateTime() in packages/fork-choice/src/forkChoice/forkChoice.ts and trace the cached-head read in produceAttestationData() at packages/beacon-node/src/api/impl/validator/index.ts. Compare the block-import and PrepareNextSlotScheduler paths named in the report; done means attestation data reflects eligible prior-slot votes even when no block arrives in the current slot.

Written by the indexing model from the issue text.