feat: add flexible BaseExecutor and BaseLeverageExecutor contracts

[Schlagonia]

Summary

This PR introduces flexible base contracts for strategies that need to perform complex operations through arbitrary calldata execution, particularly for leverage looping and borrowing activities.

Changes

• BaseExecutor: Minimal generic execution framework with customizable hooks
• BaseLeverageExecutor: Extends BaseExecutor with leverage-specific functionality
• Comprehensive interfaces for both contracts
• Documentation: Detailed README with usage examples and security guide

Architecture

BaseHealthCheck
    └── BaseExecutor (generic execution with hooks)
            ├── YourCustomStrategy (for non-leverage strategies)
            └── BaseLeverageExecutor (adds leverage features)
                    └── YourLeverageStrategy

Key Features

BaseExecutor

• Simple target whitelisting via boolean mapping
• Pre/post execution hooks for custom verification
• Batch execution support with configurable failure handling
• Executor role management for external callers
• Emergency pause mechanism

BaseLeverageExecutor

• Leverage ratio tracking and enforcement (max/target ratios)
• Automated leverage verification after execution
• Total assets monitoring with configurable loss protection
• Emergency deleveraging capabilities
• Minimum asset balance requirements

Security Features

• ✅ Strict target whitelisting (management-only configuration)
• ✅ ReentrancyGuard on all execution functions
• ✅ Customizable verification through hooks
• ✅ Executor role access control
• ✅ Emergency pause and deleverage functions
• ✅ No hardcoded routes or logic - fully flexible

Testing Checklist

• Test target whitelisting works correctly
• Test execution fails for non-whitelisted targets
• Test pause mechanism blocks all executions
• Test executor role permissions
• Test leverage limits are enforced (BaseLeverageExecutor)
• Test custom verification hooks trigger correctly
• Test batch execution with mixed success/failure scenarios
• Test emergency functions under stress conditions
• Fuzz test with random calldata to whitelisted targets

Notes

The design intentionally separates generic execution logic (BaseExecutor) from leverage-specific functionality (BaseLeverageExecutor) to provide maximum flexibility. Strategies can choose which base to inherit from based on their needs.

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