Fix hurdle_rate insensitivity in productivity investment planner

[jose-moran]

The hurdle rate was not affecting investment decisions because the profitability formula was incorrectly comparing a present value (cost_reduction * (1+r)/r) against the candidate investment, which always resulted in the PV being 50-500x larger than the investment.

Changes:

• Add is_investment_profitable() method with correct formula from investment_decision.md: (I/V)^(1-α) ≤ φ * (1+r_h)/r_h
• Use nominal production (price × physical production) for dimensional correctness - I/V must be dimensionless
• Add firm_industries parameter to get firm-specific prices
• Include unit costs check (zero unit costs = no savings possible)
• Update TFP production tests to pass pre-scaled limiting inputs, reflecting that set_targets() now pre-scales these values

Fixes #60

@sternluke please have a look and let me know if this is OK for you