PDHG add-ons

[gdalle]

• Infeasibility detection (https://arxiv.org/abs/2102.04592)
• Feasibility polishing (https://arxiv.org/abs/2501.07018)
• Halpern iteration (https://arxiv.org/abs/2407.16144)
• Better primal weight updates (https://arxiv.org/abs/2507.14051)