Allow investment caps with soft limits

[ArnoClaude]

Description

Enable soft budget constraints that penalize (not forbid) investments exceeding a threshold.

Current Behavior

• Revoletion accepts investmentmax per scenario as a hard cap
• Investments cannot exceed this value

Proposed Behavior

• User provides optional investment_soft_cap and soft_cap_penalty in config
• Investments above soft cap incur penalty in objective: penalty * (investment - soft_cap)
• Hard cap (investmentmax) remains enforced by Revoletion

Implementation Notes

• Add to objective function: soft_cap_penalty * max(0, total_investment - soft_cap)
• Requires auxiliary variable + constraint to linearize the max(0, x) term
• Penalty is cost per unit of overage (e.g., €/MW or €/€ invested)

Open Questions

• Soft cap per-technology, per-stage, or total?
• Linear penalty or quadratic (increasing marginal penalty)?
• Should penalty be stage-dependent (harder to exceed in later stages)?