test(pricing): put-call parity, CRR convergence, Greek sanity regression tests (#345)

[joaquinbejar]

Summary

Add `tests/unit/pricing/identities_test.rs` with four deterministic regression tests covering the fundamental pricing identities from `rules/global_rules.md` §Numerical Discipline:

• `put_call_parity_grid`: grid spots × strikes × IVs × tenors asserting `C - P == S - K · e^(-rT)` within `1e-4` under zero dividend.
• `crr_binomial_converges_to_black_scholes`: step schedule `N ∈ {50, 200, 800}`; binomial within 1% of BS at `N = 800` and strictly better than at `N = 50`.
• `greek_sanity_gamma_vega_equal_call_put`: `Γ_call == Γ_put` and `Vega_call == Vega_put` for European options.
• `greek_sanity_delta_call_minus_put_equals_unity`: `Δ_call − Δ_put ≈ e^{-qT} → 1` with zero dividend.

All tolerances are explicit per assertion and tests are fully deterministic (no RNG).

Monte-Carlo std-error bound test deferred to a follow-up that builds on the deterministic-RNG primitive from #344, so it can be seeded reproducibly.

Closes #345 (core identities; MC std-error follow-up tracked separately).

Test plan

• `cargo test --test tests --all-features pricing::identities_test` — 4 passed; 0 failed.
• `cargo clippy --all-targets --all-features -- -D warnings` clean.
• `cargo fmt --all --check`.