[HOLD] Advance algebraic states in MHE windows

[mickaelbegon]

Summary

• advance algebraic-state initial guesses with each receding-horizon window
• pass updated a_init values through update_initial_guess
• preserve the distinct window-selection behavior of standard, cyclic, and multi-cyclic MHE
• support collocation point selection in the multi-cyclic path
• add a regression test for shifting algebraic states and repeating the terminal value

Root cause

RecedingHorizonOptimization.advance_window() updated state, control, and parameter initial guesses, but never read Solution.decision_algebraic_states() and never forwarded a_init. Algebraic variables therefore retained stale values between windows.

Validation

• targeted regression test: 1 passed
• complete tests/shard1/test_mhe.py: 7 passed, 1 skipped
• the additional ACADOS case could not be collected locally because acados_template is not installed; the failure occurs while importing the ACADOS interface, before the modified path is executed

Closes #971

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