[HOLD] Evaluate spline initial guesses at collocation points

[mickaelbegon]

Summary

• retain the collocation sub-node index when evaluating spline initial guesses
• account for the number of collocation points in spline time normalization
• include the spline time origin instead of assuming it starts at zero
• add a focused regression test with six distinct values across two intervals

Root cause

Spline initial guesses were accepted for collocation, but all intermediate points within an interval were collapsed onto one shooting-node time. PathCondition.evaluate_at also ignored repeat and the spline time offset.

Validation

• focused spline unit tests
• existing collocation/SPLINE OCP regression
• 3 passed

Progress toward #521.

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