is_healthy is now true only if the block store is accessible and
has produced at least the genesis block.
Clients should poll chain_height twice with a delay to verify
the sequencer is making progress.
Refs #244
Previously check_health() always returned Ok(()) - completely useless.
Now returns HealthStatus { chain_height, is_healthy } so clients can:
1. Call check_health() once to get current chain_height
2. Wait a few seconds
3. Call again - if chain_height increased, sequencer is healthy
Refs #244
Previously check_health() always returned Ok(()) without checking
anything meaningful. A sequencer could be stuck and still pass.
Now check_health() returns the current chain height (BlockId).
Clients can call this twice with a delay to verify the chain is
progressing.
Refs #244
