docs(release): CHANGELOG for 0.16.0 (#346)

[joaquinbejar]

Summary

• Add `CHANGELOG.md` following Keep a Changelog 1.1.0 with Added / Changed / Fixed / Internal sections.
• Covers the full 0.15.3 → 0.16.0 migration: Eliminate Box<dyn Error> and String errors from public surface #333, Add # Errors and # Panics doc sections to public fallible functions #334, Audit Decimal arithmetic → use checked_* on monetary flows #335, Guard NaN/Inf at f64 ↔ Decimal boundaries in pricing kernels #336, Introduce NonZeroU64/NonZeroUsize for step and simulation counts #337, Apply #[must_use] across pure fns, builders, and Result-returning fns #338, Apply #[inline]/[always]/[never]/[cold] per hot-path and error-path rules #339, Audit #[repr(u8)], #[derive] order, and serde conventions #340, Replace unchecked [] indexing with .get(i).ok_or_else(..) #341, Add #[tracing::instrument] on public hot paths #342, Doc coverage audit: /// on every pub item, deny missing_docs #343, Seed deterministic RNG in all Monte-Carlo and simulation tests #344, Add put-call parity, CRR convergence, and Greek sanity regression tests #345, Add iterator-based d_sum_iter helper and eliminate Vec allocation in custom::calculate_profit_at #372.
• `Cargo.toml` version is already `0.16.0`; `README.md` regenerates identically from `README.tpl` under `cargo readme`.

Tag + GitHub release will be cut immediately after merge.

Closes #346 (changelog + housekeeping; tag creation follows the merge).

Test plan

• `cargo clippy --all-targets --all-features -- -D warnings` clean.
• `cargo fmt --all --check`.
• `cargo test --lib --all-features` — 3756 pass; 4 pre-existing `chains::chain` failures unrelated.
• `cargo readme` — no README diff.