* fix(costs): preserve sparse all-history aggregates

Separate the input-record budget from the calendar span so old model rows and daily averages remain correct. Count eligible empty dates without allocating a calendar-sized array. Update usage and cost-history guides and Unreleased notes; no TODO parity item is completed.

* fix(costs): retain gap filling for measured dates

An amountless row for a date already covered by measured amounts must not disable empty-day inference. Match the existing daily gap filler with separate blocked and measured date sets. Document the rule and cover both input orders.

* fix(usage): preserve cache and notification baselines

Wait for an authoritative quota before priming initial quota and pace notification state. Preserve the fitting cache prefix when valid providers exceed the byte budget. Add adversarial transition and publication regressions and update usage/development guides and Unreleased notes; no TODO parity entry is completed.