tubedude / finance-elixir / 6125d6828959570ece3c3483c94b8572e865871d-PR-28

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Pull #28

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Domain guards, discounting cleanup, and review fixes (1.6.1)

Correctness:
- A rate at or below -100% returns {:error, :undefined} instead of raising
  ArithmeticError or returning garbage everywhere (1 + rate)^t is computed:
  xnpv/npv/mirr, TVM fv/pv/pmt/amortization_schedule, Bonds price + risk
  metrics, Returns dpp/profitability_index/twr, effective_annual_rate, sln.
  nominal_rate(-1.0, m) now returns -m so it stays the inverse of
  effective_annual_rate at total loss.
- Bonds duration/modified_duration/convexity return :undefined when a negative
  coupon zeroes the price, rather than a nonsensical value or a crash.
- ytm and profitability_index round once at full precision (ytm's periodic
  rounding was amplified by freq); a regression test pins ytm at the default
  precision.
- xirr(dates, []) returns :mismatched_lengths instead of crashing.
- Drop the broad rescue in normalize (dates parse via Date.from_erl/1); a
  malformed amount raises as a caller bug instead of being mislabeled
  :invalid_date.

Consistency and cleanup:
- Centralize discounting in one overflow-safe (1 + rate)^-t helper
  (Shared.discount_factor), used by present_value and the bond/returns metrics,
  which previously used a divide form that could overflow.
- Validate :precision as 0..15; accept integer :guess/:tolerance. Require an
  integer Bonds freq; reject a fractional syd period.
- Bracket scan stops one step sooner when the root sits at the guess. Hoist the
  solvers' safely/1 and rounding into Shared; dedupe the amortization row
  converters; trim comments that restated code.

Testing:
- Run the solver fuzz properties against both Newton and Brent, and add a
  dated, many-flow, random-sign XIRR property (only 2-flow dated series were
  fuzzed before). A real Newton/Brent robustness bug would surface here.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AVibXT2w1dEdVtMHqffFKn

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