Support purely backward looking models (VAR, nonlinear growth models)#214

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copilot/add-backward-model-functionality

Commits on Jan 1, 2026

Add newton algorithm for backward looking models with residual and jacobian functions

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Fix newton state_update to add SS before evaluation and subtract after

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Add simulate_newton function for backward looking models with custom initial states

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Integrate initial_state in levels into existing API for backward looking models with newton

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Add unit root model support and fix SS equation parsing for backward looking models

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Refactor backward looking model detection: rename has_unit_root to has_valid_ss_for_reference and fix tests

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Commits on Jan 2, 2026