budgetTracker: Add onOverspend callback when spending exceeds budget ceiling

[Kingsman-99]

Description

budgetTracker tracks cumulative spending against a budget but silently exceeds limits. Contributors should add an onOverspend(overspentBy: number, budgetId: string) callback option that fires whenever a new payment pushes the total above the configured budget ceiling, so callers can trigger warnings or block further payments.

Acceptance Criteria

• An onOverspend callback option is accepted by the tracker's configuration
• The callback fires exactly once per payment that crosses the budget ceiling (not once per subsequent payment)
• overspentBy is the positive difference between the new total and the budget limit
• Existing tests continue to pass

Context

• Target file: src/lib/budgetTracker.ts