v0.17.2 — Black-76 + Garman–Kohlhagen pricing models

Release adding two new closed-form pricing models:

• Black-76 (Black 1976) for European options on futures and forwards.
• Garman–Kohlhagen (1983) for European FX options.

0.17.0 and 0.17.1 were preparatory iterations of this work
(0.17.0 was never published; 0.17.1 shipped to crates.io with a
partial subset). 0.17.2 is the first version that ships both models
together. PricingEngine is #[non_exhaustive] (semver-major from the
0.16.x line) and the two new variants are appended at the tail of the
enum so existing discriminants are preserved.

Added

Black-76 model (Black 1976):

• pricing::black_76: closed-form black_76(option) -> Result<Decimal, PricingError>
  for European options on futures / forwards. Reuses the existing d1
  / d2 / big_n helpers; Decimal end-to-end via d_mul / d_sub;
  tracing::instrument on the entry point. Only OptionType::European
  is supported — American, Bermuda and exotics return
  PricingError::UnsupportedOptionType.
• pricing::Black76 trait with default calculate_price_black_76
  (mirrors BlackScholes).
• pricing::PricingEngine::ClosedFormBlack76 variant + dispatch from
  price_option.
• greeks::utils::calculate_d_values_black_76 pub(crate) helper.
• examples/examples_pricing/src/bin/black_76.rs: runnable demo
  (Hull canonical example, ITM commodity-futures call, unified-API
  dispatch, short-side sign convention).

Garman–Kohlhagen model (Garman & Kohlhagen 1983):

• pricing::garman_kohlhagen: closed-form
  garman_kohlhagen(option) -> Result<Decimal, PricingError> for
  European options on FX spot rates. Structurally identical to BSM
  with q = r_f; the implementation delegates to black_scholes
  after type validation, guaranteeing bit-exact equivalence (verified
  to 1e-9 in the tests).
• pricing::GarmanKohlhagen trait with default
  calculate_price_garman_kohlhagen (mirrors the BlackScholes
  trait pattern).
• pricing::PricingEngine::ClosedFormGK variant + dispatch from
  price_option.
• examples/examples_pricing/src/bin/garman_kohlhagen.rs: runnable
  demo (Hull canonical USD/GBP, ITM EUR/USD with FX parity check,
  unified-API dispatch, symmetric-rate degenerate case).

Infrastructure updates:

• examples/examples_pricing/: new workspace member with binaries for
  both models.
• lib.rs mermaid: Forward-Priced subgraph routing
  black_76 -> {Future, Forward}; new FX / Currency subgraph routing
  garman_kohlhagen -> FX Spot.

Changed

• pricing::PricingEngine is now #[non_exhaustive] so future engine
  variants do not require a new major bump.
• pricing::mod.rs Core Models / Model Selection Guidelines /
  Performance Considerations now include both Black-76 and
  Garman–Kohlhagen with explicit field mapping documentation.
• financial_types bumped to 0.2.2 (adds UnderlyingAssetType::Future
  and UnderlyingAssetType::Forward).
• PricingError and GreeksError pass-through in closed-form dispatch
  (BS, Black-76, GK) for full error-variant fidelity.