On hold — discussion with @EveCharbie required — Reduce unused RK state symbols

[mickaelbegon]

Summary

• expose whether an ODE solver consumes only the starting state/algebraic-state CX
• omit the unused middle CX for explicit RK solvers with PhaseDynamics.ONE_PER_NODE
• keep shared dynamics, collocation, trapezoidal integration, and controls unchanged
• add a regression test covering the reduced state layout and unchanged control layout

This is a focused first performance improvement toward #966. For explicit RK phases using one dynamics function per node, state-like symbolic columns are reduced from three to two at every node while preserving independent start/end inputs required by continuity.

Tests

• pytest tests/shard2/test_global_getting_started.py::test_rk_one_per_node_does_not_declare_unused_state_cx -q
• pytest tests/shard2/test_global_getting_started.py::test_pendulum[...] -q (RK4, SX, one thread, ONE_PER_NODE, IPOPT; existing numerical regression)

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