Add Black-76 pricing model for futures/forwards (#396) (#398)

* feat(pricing): add Black-76 model for options on futures and forwards

Implement Black-76 closed-form pricing for European options on futures,
forwards, swaptions, and caps/floors. Core formulas:
- d1 = [ln(F/K) + σ²T/2] / (σ√T)
- d2 = d1 - σ√T
- Call: e^(-rT) * [F*N(d1) - K*N(d2)]
- Put: e^(-rT) * [K*N(-d2) - F*N(-d1)]

Key differences vs Black-Scholes:
- Input is forward price F (not spot S)
- No carry term (F already incorporates all carry)
- Unified discount factor e^(-rT) on both legs

Changes:
- src/pricing/black_76.rs: kernel + trait + 20 unit tests
- src/greeks/utils.rs: calculate_d_values_black_76 helper (b=0 drift)
- src/pricing/mod.rs: module documentation and exports
- src/pricing/unified.rs: PricingEngine::ClosedFormBlack76 variant + dispatch
- examples/examples_pricing/: new workspace crate with runnable demo
- Cargo.toml: workspace members (alphabetical sort)

Tests validate:
- Hull canonical reference (F=K=20, ATM)
- Put-call parity across ATM/ITM/OTM scenarios
- Equivalence to BS-Merton with S=F*e^(-rT), q=0
- Monotonicity in forward price
- Short side sign negation
- Quantity invariance
- Unsupported types (American, Bermuda, exotics)

All tests pass; clippy/fmt/build clean.

* docs(pricing): list Black-76 in lib.rs/pricing module guides

Adds Black-76 to the file inventory in `src/lib.rs::Pricing Models`,
extends the pricing-models mermaid with a `Forward-Priced` subgraph
mapping `black_76 -> {Future, Forward}`, and lists Black-76 in the
`pricing/mod.rs` Model Selection Guidelines and Performance
Considerations sections.

* fix(pricing): add #[non_exhaustive] to PricingEngine enum for semver compatibility

The new ClosedFormBlack76 variant was flagged as a semver-breaking change
because PricingEngine is a public exhaustive enum. Mark it #[non_exhaustive]
to signal that future variants are possible, and add a catch-all _ pattern
to price_option dispatch.

* release(0.17.0): bump major + address copilot review

Bumps the crate version to 0.17.0 because the 0.16.x → main diff
introduces two semver-breaking changes: `PricingEngine` is now
`#[non_exhaustive]` and `PricingEngine::ClosedFormBlack76` shifts the
implicit discriminant of `PricingEngine::MonteCarlo` from 1 to 2
(flagged by `cargo-semver-checks`).

Also addresses the three Copilot review comments on PR #398:
- pricing/mod.rs: reword the Black-76 module description so it lists
  "options on" futures / forwards / commodity futures rather than the
  underlyings themselves.
- pricing/unified.rs: extend the `price_option` `# Errors` docstring to
  cover Black-76 (`MethodError` for zero-vol / non-finite,
  `UnsupportedOptionType` for non-European inputs).
- greeks/utils.rs: compute `expiration_date.get_years()?` once in
  `calculate_d_values_black_76` and reuse the value for both `d1` and
  `d2`.

Also drops the now-unreachable `_` arm in `price_option` (intra-crate
matches see all variants regardless of `#[non_exhaustive]`), which was
failing the `lint` job, and updates the version references in
`src/lib.rs` and `CHANGELOG.md`.

Author: joaquinbejar

CHANGELOG.md

@@ -7,6 +7,43 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+## [0.17.0] - 2026-04-26
+
+Major release adding the Black-76 closed-form pricing model for European
+options on futures and forwards. The bump to `0.17.0` is required because
+`PricingEngine` is now `#[non_exhaustive]` (a semver-breaking change for
+downstream exhaustive matches) and the addition of
+`PricingEngine::ClosedFormBlack76` shifts the implicit discriminant of
+`PricingEngine::MonteCarlo` from `1` to `2`.
+
+### Added
+
+- `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/`: new workspace member with binary
+  `black_76` (Hull canonical example, ITM commodity-futures call,
+  unified-API dispatch, short-side sign convention).
+- `lib.rs` mermaid: `Forward-Priced` subgraph routing
+  `black_76 -> {Future, Forward}`.
+
+### 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 Black-76.
+- `financial_types` bumped to `0.2.2` (adds `UnderlyingAssetType::Future`
+  and `UnderlyingAssetType::Forward`).
+
 ## [0.16.5] - 2026-04-20
 
 Documentation-only release. Refresh the crate-level rustdoc and

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "optionstratlib"
-version = "0.16.5"
+version = "0.17.0"
 edition = "2024"
 authors = ["Joaquin Bejar <jb@taunais.com>"]
 description = "OptionStratLib is a comprehensive Rust library for options trading and strategy development across multiple asset classes."
@@ -101,17 +101,18 @@ crate-type = ["cdylib", "rlib"]
 
 [workspace]
 members = [
+    "examples/examples_chain",
+    "examples/examples_curves",
+    "examples/examples_exotics",
+    "examples/examples_metrics",
+    "examples/examples_pricing",
     "examples/examples_simulation",
+    "examples/examples_strategies",
     "examples/examples_strategies_best",
     "examples/examples_strategies_delta",
-    "examples/examples_strategies",
+    "examples/examples_surfaces",
     "examples/examples_visualization",
-    "examples/examples_chain",
     "examples/examples_volatility",
-    "examples/examples_curves",
-    "examples/examples_surfaces",
-    "examples/examples_metrics",
-    "examples/examples_exotics",
 ]
 
 [workspace.dependencies]