* fix(usage): correct quota ranges and export completeness

* fix(share): preserve partial model cost evidence

Track token observations with unknown cost through model aggregation and treat exhausted model-day scans as source truncation. Cover both cases in normal and private exports.

* fix(share): retain exact legacy history tail

Limit source-scan partial notices to calendar-anchored history. The legacy tail intentionally ignores older rows, so its ranking remains complete when those rows are outside the requested period.