feat(core): Add non-breaking flow execution statistics collection

[madawei2699]

This PR introduces a new flow execution statistics feature without breaking existing APIs. It is the first phase of implementing execution limits and monitoring capabilities.

Key Changes

• Add ExecutionStats module for tracking flow execution metrics
• Integrate statistics collection within Flow module with zero API changes
• Track important metrics:
  • Steps executed
  • Signal type frequencies
  • Handler call frequencies
  • State size over time
  • Execution duration
• Add tests covering both basic functionality and edge cases
• All statistics are collected automatically for every flow execution
• Statistics can be accessed via Flow.get_last_execution_stats/0

Future Enhancements

This lays the foundation for future phases:

• Flow execution limits and constraints
• Performance monitoring and optimization
• Debugging and tracing tools

The changes are implemented in a backward-compatible manner, ensuring existing code continues to work without modification while enabling new monitoring capabilities.

[coveralls]

Pull Request Test Coverage Report for Build 3ff6c10190ba671e234d76aebc6471aabc2523d8-PR-4

Details

• 29 of 29 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+1.5%) to 89.489%

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Totals
Change from base Build 1851f3c01f2527c0a0de27dbafae060c0cef189a:	1.5%
Covered Lines:	315
Relevant Lines:	352

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💛 - Coveralls