Estimate network hashrate from observed block work

[zpalmtree]

What changed

• estimate recent network hashrate as the sum of sampled block difficulty divided by sampled positive block time
• retain the existing average block-time output
• add a regression test with changing block difficulty

Why

The previous calculation multiplied the average sampled block rate by the next block difficulty. When difficulty changed within the sample window, that mixed two different windows and biased the displayed network hashrate. Each sampled interval already has an exact amount of observed work: the difficulty of the block that ended it.

This is an explorer/statistics correction only. It does not change consensus, difficulty adjustment, block validation, or mining templates.

Validation

• go test ./...