ENH: derive constrained equations of motion for udot_rail2 tip-off phase

Complete the 3-DOF single-rail-button (tip-off) phase from issue #28.

- Fix the phase transition ordering: rail1 -> udot_rail2 (at effective_1rl,
  upper button exit) -> u_dot_generalized (at effective_2rl, lower button
  exit). Previously the thresholds were swapped, so udot_rail2 was inserted
  after free flight and never exited.
- Replace the placeholder udot_rail2 (which reused free-flight dynamics with an
  ad-hoc velocity projection) with rigorous constrained dynamics: the lower
  button slides along the fixed rail while roll is suppressed. The reaction
  wrench (normal force + roll moment) is solved from a 3x3 linear system so the
  button's perpendicular acceleration and the roll acceleration vanish, derived
  in the true body frame on top of the validated u_dot_generalized solution.
- Make the feature opt-in (use_udot_rail2 defaults to False); disabled runs are
  bit-for-bit identical to previous behavior.
- Factor the rail-exit root finding into a shared helper.
- Rewrite the unit tests to check phase ordering, the opt-in default, the
  on-rail constraint (button stays on the rail to machine precision), zero
  roll, and the gravity tip-off direction.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Gui-FernandesBR

CHANGELOG.md

@@ -32,6 +32,7 @@ Attention: The newest changes should be on top -->
 
 ### Added
 
+- ENH: 3-DOF single rail button flight phase (tip-off analysis) [#920](https://github.com/RocketPy-Team/RocketPy/pull/920)
 - ENH: update master with develop [#1081](https://github.com/RocketPy-Team/RocketPy/pull/1081)
 ### Changed