[Lagrangian.Solver] UnbuiltConstraintSolver: implement hotstart

[fredroy]

Based on

• [Lagrangian.Solver] UnbuiltConstraintSolver: Fix resetForUnbuiltResolution (constraint re-ordering) #5871

in #5871 (and more cases) it was noted that (unbuilt) LCP was always faster than UnbuiltGaussSeidelConstraintSolver (in average ~10-20 %) and usually converged in less iterations.
Even if the algorithm is rigorously the same (apparently...)

After investigations, Claude 🤖successfully found that the main difference is UnbuiltGaussSeidelConstraintSolver was always starting its GS with the forces at 0 (cold start), whereas LCP was keeping track of the previous forces.
It also provided the implementation (I could never do that by myself 💩) which... looks good to me at least 🙃

Benches (continuation of #5871)

• 5000 steps on 3instru_collis (deformable), wire_optimization on

LCP                : 5000 iterations done in 43.9782 s ( 113.693 FPS)
UGS (w/o hotstart) : 5000 iterations done in 49.5163 s ( 100.977 FPS)
UGS (w hotstart)   : 5000 iterations done in 44.0242 s ( 113.574 FPS)

• for the fun, wire_optimization off

LCP                : 5000 iterations done in 63.7577 s ( 78.4219 FPS)
UGS (w/o hotstart) : 5000 iterations done in 99.2367 s ( 50.3846 FPS)
UGS (w hotstart)   : 5000 iterations done in 62.5122 s ( 79.9844 FPS)

This PR could be considered a little bit... controversial (:vibe:) as Claude-code implemented lots of stuff

And a last question would be, either d_initialGuess set to True or False by default ? (false being the behavior as before)

[with-all-tests]

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[fredroy]

[ci-build][with-all-tests]