Eliminate mixins and move to pure service architecture

[astrogilda]

Problem

The codebase currently uses an inconsistent hybrid approach with both mixins and services, creating architectural complexity and maintenance burden.

Issues with Current Architecture

1. Fragile Mixin Dependencies: Mixins like StationarityMixin blindly assume host classes have certain attributes (self.residuals) without explicit contracts

2. Multiple Inheritance Complexity: Diamond inheritance patterns create MRO issues and unpredictable behavior

3. Single Responsibility Violations: Classes like BaseTimeSeriesBootstrap (737 lines) and TimeSeriesModel (770 lines) violate SRP by handling validation, serialization, sklearn compatibility, bootstrap logic, and state management

4. Inconsistent Pattern Usage: Some functionality uses services, others use mixins, creating confusion about architectural direction

Concrete Examples

Hidden Dependencies in Mixins - StationarityMixin assumes self.residuals exists
Complex Multiple Inheritance - WholeResidualBootstrap inherits from ModelBasedBootstrap AND WholeDataBootstrap
God Object Pattern - TimeSeriesModel has 770 lines doing everything

Solution: Pure Service Architecture

Move to a 100% service-based architecture with explicit dependencies and single inheritance only.

Benefits

1. Explicit Dependencies: Services declare exactly what they need
2. Easy Testing: Each service can be tested in isolation
3. Clear Contracts: No hidden assumptions about host class structure
4. Single Inheritance: Eliminates MRO complexity
5. Modular Composition: Features composed through service injection

Migration Plan

Phase 1: Extract Mixin Functionality to Services

• StationarityMixin → StationarityTestingService
• WholeDataBootstrap → WholeDataResamplingService
• BlockBasedBootstrap → BlockResamplingService

Phase 2: Eliminate Multiple Inheritance

• Each bootstrap class inherits only from BaseTimeSeriesBootstrap
• Behavior differences achieved through service composition
• Update all concrete bootstrap implementations

Phase 3: Decompose Monoliths

• Break down TimeSeriesModel into focused services
• Simplify BaseTimeSeriesBootstrap to pure orchestration

Phase 4: Update Tests and Documentation

• Update all tests to work with new service architecture
• Update documentation to reflect architectural changes
• Add migration guide for users

Success Criteria

• Zero mixins in the codebase
• All classes use single inheritance only
• Service container pattern used throughout
• All tests pass with new architecture
• Performance benchmarks show no regression
• Clear migration path for users

This refactoring will result in a much more maintainable, testable, and understandable codebase that follows clear architectural principles.