docs: add refactoring plan for extracting complex methods (#182)

Author: astrogilda

refactoring-plan-182.md

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+# Refactoring Plan for Issue #182: Extract Complex Methods
+
+## Overview
+This PR addresses the complexity in `TimeSeriesModel.fit_arch` method by extracting it into smaller, focused methods.
+
+## Changes to be implemented:
+1. Extract GARCH model creation into `_create_garch_model()`
+2. Extract EGARCH model creation into `_create_egarch_model()`
+3. Extract TGARCH model creation into `_create_tgarch_model()`
+4. Extract common parameter validation into `_validate_arch_params()`
+
+## Files affected:
+- `src/tsbootstrap/time_series_model.py`
+
+## Testing plan:
+- Ensure all existing tests pass
+- Add unit tests for each extracted method
+- Verify model creation behavior remains unchanged