v0.8.1

#67 Fix state preservation in graph.Interrupt()

Fixed a critical bug where state modifications made before calling
graph.Interrupt() were lost. Now state updates are correctly preserved
in GraphInterrupt.State and passed to resume operations.

Changes:
- executeNodeWithRetry: Return node result with NodeInterrupt error
- executeNodesParallel: Save results even when NodeInterrupt occurs
- InvokeWithConfig: Merge state before checking for interrupt errors
- Added comprehensive test cases for value-type and map-type states
- Added payment_interrupt example demonstrating the fix

This ensures that transaction IDs, payment statuses, and other state
modifications persist correctly across interrupt/resume cycles.