Add Stokes/Poiseuille 1D FD playground: temporal convergence for index-1 semi-explicit DAE with constraint lifting, differentiated-constraint remedy, coupled block-solve exploration, and monolithic RADAU-IIA solver#641

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copilot/test-temporal-order-convergence-again

Commits on Mar 26, 2026

Redesign: use genericImplicitConstrained sweeper with MeshDAE state

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Implement Stokes/Poiseuille DAE playground with SemiImplicitDAE sweeper

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Add constraint-lifting variant stokes_poiseuille_1d_fd_lift; update run_convergence.py to compare both

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Use nu=0.1, Tend=1.0 for clean M+1=4 convergence orders; document U-formulation order limit

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Add FullyImplicitDAE-compatible Stokes classes; update run_convergence.py with 4-variant comparison

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Add differentiated-constraint classes achieving full 2M-1 RADAU order for velocity and pressure

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Fix diffconstr order claim: M+2 (not 2M-1); for M=3 M+2=5=2M-1 is coincidental

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Add stokes_poiseuille_1d_fd_coupled: (N+1)x(N+1) block solve with optional projection

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Add run_monolithic.py: monolithic RADAU-IIA collocation (algebraic and differentiated-constraint variants)

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