Session resume now restores accumulated cost, tracked separately

What

• When resuming a session with a history_file, the resume path now reads and restores a restoredCostUsd value from that file instead of ignoring cost on resume.
• The cost-tracking snapshot/restore mechanism now stores a distinct restoredCostUSD value alongside hasUnknownModelCost (previously both were packed into the same internal field), and reset() now explicitly zeroes the restored value.
• The cost-tracking class gained a restoredCostUSD() accessor and internal counter, and the cost-state schema's totalCostUSD now has a floor instead of being unconstrained.

Why

Restoring accumulated cost correctly when resuming a session keeps the cost totals shown to the user accurate across resumes, instead of resetting or being conflated with the separate tracking of cost from an unknown model.