refactor(series,greeks): eliminate .unwrap()/.expect() (#320)

[joaquinbejar]

Summary

• Removes every production .unwrap() / .expect() from src/series/ and src/greeks/ per the M1 — Panic-Free Core milestone (22 sites total across 3 files).
• Makes generator_optionseries fallible (Vec<Step<...>> → Result<Vec<Step<...>>, ChainError>), matching the precedent set by PR M1: Eliminate .unwrap()/.expect() from src/chains/ (#317) #348 for generator_optionchain / generator_positive and aligning with the fallible RandomWalk::new / Simulator::new constructors from PR fix(simulation,pricing): fallible RandomWalk/Simulator constructors + Telegraph guard (#349, #351) #352.
• Replaces the runtime fallible (Decimal::TWO * PI).sqrt().unwrap() in n(x) with a precomputed 1/sqrt(2π) constant; collapses big_n(x)'s redundant is_none() + unwrap() pair into a let-else.
• Refactors the OptionSeries serde Serialize::serialize chain-map closure to surface inner errors via serde::ser::Error::custom instead of panicking.
• Init-invariant fix in generator_optionseries Historical branch: empty / insufficient prices now return Ok(vec![init_step]) instead of vec![], matching the contains-init-step contract used by the rest of the chain-generator family.

Per-pattern transformations

Before	After
walker.brownian(walk_params).unwrap() (and 8 siblings)	walker.brownian(walk_params)? (after sig change)
(Decimal::TWO * PI).sqrt().unwrap()	dec!(0.3989422804014326779399461) precomputed 1/sqrt(2π) constant
Decimal::from_f64(-11.7).unwrap()	dec!(-11.7)
Normal::new(0.0, 1.0).unwrap() (infallible by construction)	.map_err(|e| DecimalError::ConversionError { ... })?
x_f64.unwrap() after is_none() early-return	let Some(x_f64) = x.to_f64() else { return Err(...); };
date.get_date_string().unwrap() in serde closure	for loop with .map_err(serde::ser::Error::custom)?
Doc-comment .unwrap() examples	# fn run() -> Result<(), Box<dyn Error>> shim with ?

Test plan

• Acceptance grep returns 0 — awk '/#\[cfg\(test\)\]/{exit} {print}' <file> \| grep -E '\.unwrap\(\)\|\.expect\(' across src/series/**/*.rs and src/greeks/**/*.rs
• cargo clippy --all-targets --all-features --workspace -- -D warnings
• cargo fmt --all --check
• cargo test --all-features --workspace (3724 lib + 416 integration + plotly)
• cargo build --release
• make pre-push (fix + fmt + lint-fix + test + readme + doc)
• Updated test_generator_optionseries_historical_empty_prices and _insufficient_prices to reflect the new contains-init-step contract (steps.len == 1 instead of is_empty)

Public-surface notes

• generator_optionseries: signature change from Vec<Step<...>> to Result<Vec<Step<...>>, ChainError>. Breaking minor (acceptable on 0.16.x — same precedent as PR M1: Eliminate .unwrap()/.expect() from src/chains/ (#317) #348 / fix(simulation,pricing): fallible RandomWalk/Simulator constructors + Telegraph guard (#349, #351) #352). Only one in-tree caller (examples/examples_simulation/src/bin/random_walk_build_series.rs), updated in this PR.
• n(x) and big_n(x) keep the same signature; behaviour unchanged on the happy path (the precomputed constant is verified to 24+ decimal places against the existing PDF reference tests).
• OptionSeries serde Serialize impl signature unchanged; behaviour now surfaces date-formatting errors as serde errors instead of panicking.

Closes #320

[Copilot]

Pull request overview

This PR advances the “Panic-Free Core” milestone by removing production .unwrap()/.expect() usage in the series and greeks modules, propagating typed errors instead of panicking, and aligning series generation with existing fallible generator patterns used elsewhere in the simulation stack.

Changes:

• Make generator_optionseries fallible (Result<..., ChainError>) and propagate errors via ?, with an init-step-preserving behavior for empty/insufficient historical data.
• Replace fallible Decimal sqrt usage in n(x) with a precomputed 1/sqrt(2π) constant; simplify big_n(x) conversion handling and surface Normal::new construction errors.
• Refactor OptionSeries custom serde serialization to convert date-formatting failures into serde errors instead of panicking.

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 3 comments.

File	Description
src/series/model.rs	Refactors OptionSeries serialization to surface date formatting errors via serde::ser::Error::custom.
src/series/generators.rs	Removes unwraps by making generator_optionseries return Result and propagating underlying failures; adjusts empty historical behavior to return init step.
src/greeks/utils.rs	Removes unwraps in PDF/CDF helpers, adds a precomputed normalisation constant, and improves conversion/error plumbing in big_n.
examples/examples_simulation/src/bin/random_walk_build_series.rs	Updates example to pass the now-fallible generator_optionseries directly into RandomWalk::new.

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[codecov]

Codecov Report

❌ Patch coverage is 81.57895% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/series/generators.rs	84.00%	4 Missing ⚠️
src/greeks/utils.rs	66.66%	3 Missing ⚠️

Files with missing lines	Coverage Δ
src/series/model.rs	78.94% <100.00%> (+0.18%)	⬆️
src/greeks/utils.rs	82.65% <66.66%> (-2.77%)	⬇️
src/series/generators.rs	89.23% <84.00%> (-3.42%)	⬇️

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[joaquinbejar]

Thanks for the review. All three findings are addressed in 387ee94.