Heavy-scan token bands rebalanced: 40M / 80M thresholds
Heavy-scan billing now kicks in earlier and scales with scan depth: the first credit covers up to 40 million tokens (was 50M), a second credit is consumed once a scan passes 40 million tokens, and a third once it passes 80 million — no scan ever costs more than 3 credits. Most scans still finish within their first credit. If a scan fails, the base credit is still refunded; band charges reflect analysis work already performed.
