refac: clarify lifetime field semantics — periods not counts

[FBumann]

Problem

Investment.lifetime and PiecewiseInvestment.lifetime are typed as int | None and documented as "Periods active after build." The implementation uses period indices (p_idx < t_idx + lt_int), so the value represents a count of periods.

However, periods themselves have durations (e.g., 5 years each). A user seeing lifetime=20 would naturally assume 20 years, but it actually means 20 periods. If periods are 5 years, that's 100 years.

Current behavior

# 3 periods of 5 years each
# lifetime=2 → active for 2 periods (10 years), NOT 2 years
Investment(lifetime=2, ...)

Proposed change

Use the period values (which are typically years) instead of counts. The lifetime integer should represent the same unit as the period index values.

# periods = [2025, 2030, 2035] (5-year periods)
# lifetime=10 → active for 10 years → 2 periods
Investment(lifetime=10, ...)

This way lifetime=20 means "20 years" when periods are in years, which matches user intuition.

Implementation

The active logic in _constrain_investment / _constrain_piecewise_investment currently does:

if t_idx <= p_idx < t_idx + lt_int:  # counting period indices

Would change to compare against actual period values/durations instead of index arithmetic.