refactor(slice-safety): deny(clippy::indexing_slicing) + scoped escapes (#341)

[joaquinbejar]

Summary

• Enforce `#![deny(clippy::indexing_slicing)]` crate-wide in `src/lib.rs`.
• Tests stay permissive via `#![cfg_attr(test, allow(clippy::indexing_slicing))]` — unit/integration tests routinely index into just-pushed buffers and are not expected to migrate.
• Migrate the highest-risk unchecked indexing in production paths to `.get(..).ok_or_else(..)` returning typed errors: `OptionChain` file-name parsing and CSV row reads in `src/chains/chain.rs`, and the binomial-root lookup in `Option::binomial_price` in `src/model/option.rs`.
• All remaining production call sites carry a scoped `#![allow(clippy::indexing_slicing)]` at the top of each affected module with a one-paragraph migration note; these are tracked as per-module follow-ups so the deny-level lint rolls out without one massive risky PR.

Closes #341 (slice-safety floor; per-module migration continues).

Test plan

• `cargo clippy --lib --all-features --all-targets -- -D warnings` clean.
• `cargo build --lib --all-features` clean.
• `cargo test --lib --all-features` — 3753 pass; 4 pre-existing unrelated `chains::chain` failures (also fail on `main`).
• `cargo fmt --all --check`.