Improve NPV summation accuracy and validate discount rates

[j-bryan]

• Use math.fsum in dcaf.metrics.npv() to preserve small residuals when large offsetting cashflows cause cancellation.
• Validate discount rates as finite and greater than -1, and document the shared contract for CashFlowStream.npv() and GenerationStream.discounted_sum().
• Cover cancellation, supported negative rates, domain boundaries, and non-finite inputs across the shared primitive and public stream APIs.

Validation lives in the shared NPV primitive so every consumer enforces the same real-valued rate domain. math.fsum improves reduction accuracy without introducing decimal arithmetic or removing support for finite rates above -1.

Closes #4
Closes #8

[dylanjm]

Changes are good, documentation is updated, and will catch edge cases with discount rates. Approved to merge.