Apply #[must_use] across pure fns, builders, and Result-returning fns

[joaquinbejar]

Overview

Apply #[must_use] across the public surface per the rules: every pure
function (pricing, greeks, validation, parsing, conversions), every builder
method, and every Result-returning function whose outcome the caller MUST
handle (PricingResult, StrategyResult, PositionResult).

Context

• rules/global_rules.md §Compiler Attributes — §#[must_use].
• Builder methods should use
  #[must_use = "builders do nothing unless .build() is called"].

Tasks

• Audit every public fn; add #[must_use] to pure accessors, pricing /
  greek computations, new / try_new / build methods, and
  Result-returning functions on critical paths
• Builder method messages per rules

Acceptance criteria

• cargo clippy -- -W must_use_candidate -W must_use_unit reports no
  high-signal omissions (project-specific allow-list documented)
• cargo clippy --all-targets --all-features --workspace -- -D warnings clean
• cargo test --all-features --workspace clean

References

• rules/global_rules.md — §Compiler Attributes