fix(simulation,pricing): fallible RandomWalk/Simulator constructors + Telegraph guard (#349, #351)

[joaquinbejar]

Summary

• Make Simulator::new and RandomWalk::new accept fallible generators #349 — RandomWalk::new and Simulator::new are now fallible, accepting a generator F: Fn(&WalkParams<X, Y>) -> Result<Vec<Step<X, Y>>, E> and returning Result<Self, E>. Removes the |p| generator_positive(p).unwrap() wrappers introduced in M1: Eliminate .unwrap()/.expect() from src/chains/ (#317) #348 across strategy tests, integration tests, and examples.
• Fix lambda_dt branch in TelegraphProcess::next_state (always-true guard) #351 — Fixes the inverted underflow guard in TelegraphProcess::next_state. Previously if lambda_dt < dec!(11.7) was always true (lambda_dt is non-positive for valid inputs), forcing probability = 1.0 every step and making the chain a deterministic alternation. Now guards against the underflow region with if lambda_dt < dec!(-11.7) and computes the standard 1 - exp(-lambda * dt) Poisson transition otherwise.
• Adds From<ChainError> for SimulationError (symmetric with the existing reverse impl) so chain-generator errors propagate naturally through simulation contexts.

Test plan

• cargo clippy --all-targets --all-features --workspace -- -D warnings
• cargo fmt --all --check
• cargo test --all-features --workspace (3724 lib + 416 integration + plotly + static_export combos)
• cargo build --release + workspace examples
• Acceptance grep — no generator_positive(...).unwrap() / generator_optionchain(...).unwrap() wrappers in RandomWalk::new / Simulator::new invocations across src/, tests/, examples/
• Telegraph regression — new test test_next_state_empirical_flip_rate_matches_poisson asserts empirical flip rate at (lambda=0.5, dt=0.01) matches 1 - exp(-0.005) ≈ 0.00499 within 5σ over 100k samples (vs. previous buggy 1.0)
• Negative tests for RandomWalk::new and Simulator::new verifying error propagation and short-circuit behaviour

Public-surface notes

• RandomWalk::new and Simulator::new change return type from Self to Result<Self, E> with a new generic error parameter. Breaking minor — same precedent as PR M1: Eliminate .unwrap()/.expect() from src/strategies/ (#316) #347 (strategy ::new) and PR M1: Eliminate .unwrap()/.expect() from src/chains/ (#317) #348 (chain generators), still on the 0.16.x line.
• TelegraphProcess::next_state signature unchanged; output distribution corrected (now actually obeys the documented Poisson semantics).

Closes #349
Closes #351

[codecov]

Codecov Report

❌ Patch coverage is 81.81818% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/error/simulation.rs	0.00%	2 Missing ⚠️

Files with missing lines	Coverage Δ
src/chains/generators.rs	84.37% <ø> (ø)
src/pricing/monte_carlo.rs	73.17% <ø> (ø)
src/pricing/telegraph.rs	81.35% <100.00%> (+0.84%)	⬆️
src/simulation/randomwalk.rs	56.60% <100.00%> (+0.83%)	⬆️
src/simulation/simulator.rs	88.31% <100.00%> (+0.31%)	⬆️
src/strategies/long_call.rs	70.88% <ø> (ø)
src/strategies/long_put.rs	71.65% <ø> (ø)
src/strategies/short_call.rs	74.00% <ø> (ø)
src/strategies/short_put.rs	72.36% <ø> (ø)
src/error/simulation.rs	12.00% <0.00%> (-1.05%)	⬇️

... and 1 file with indirect coverage changes

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[Copilot]

Pull request overview

Updates the simulation APIs to propagate generator errors instead of panicking, and fixes an always-true underflow guard in TelegraphProcess::next_state so transition probabilities follow the intended Poisson semantics.

Changes:

• Make RandomWalk::new and Simulator::new fallible (Result<_, E>) by accepting fallible generators and propagating generator errors.
• Fix TelegraphProcess::next_state underflow guard (lambda_dt < -11.7) and add an empirical regression test validating the flip rate.
• Update in-tree tests/examples to remove unwrap()/expect() generator wrappers and use ? / explicit Ok(...) handling; add From<ChainError> for SimulationError.

Reviewed changes

Copilot reviewed 13 out of 25 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/unit/simulation/model_and_randomwalk_tests.rs	Updates unit tests for fallible constructors (but introduces a mispositioned inner attribute).
tests/unit/pricing/unified_pricing_test.rs	Converts Monte Carlo tests to return Result and propagates simulator construction errors.
tests/unit/chain/random_walk_chain.rs	Updates chain random-walk generator to be fallible and propagate errors via ?.
src/strategies/short_put.rs	Updates strategy simulation tests to use fallible Simulator::new without generator unwrap().
src/strategies/short_call.rs	Same as above for short call strategy tests.
src/strategies/long_put.rs	Same as above for long put strategy tests.
src/strategies/long_call.rs	Same as above for long call strategy tests.
src/simulation/simulator.rs	Makes Simulator::new fallible; propagates generator errors; adds short-circuit regression test.
src/simulation/randomwalk.rs	Makes RandomWalk::new fallible; adds error-propagation regression test; updates unit tests.
src/pricing/telegraph.rs	Fixes transition probability guard and adds empirical flip-rate regression test.
src/pricing/monte_carlo.rs	Updates Monte Carlo tests to handle fallible simulator construction.
src/error/simulation.rs	Adds From<ChainError> for SimulationError to ease error propagation.
src/chains/generators.rs	Updates chain generator tests to use fallible RandomWalk::new.
examples/examples_simulation/src/bin/unified_pricing.rs	Updates demo generator and example main to propagate constructor errors (?).
examples/examples_simulation/src/bin/strategy_simulator.rs	Updates simulator construction to propagate errors (?).
examples/examples_simulation/src/bin/simulator.rs	Updates simulator construction to propagate errors (?).
examples/examples_simulation/src/bin/short_put_strategy_simulation.rs	Updates simulator construction to propagate errors (?).
examples/examples_simulation/src/bin/short_put_simulation.rs	Updates simulator construction to propagate errors (?).
examples/examples_simulation/src/bin/random_walk_chain.rs	Updates random-walk construction to propagate errors (?).
examples/examples_simulation/src/bin/random_walk_build_series.rs	Wraps infallible series generator to match new fallible constructor signature.
examples/examples_simulation/src/bin/random_walk_build_chain.rs	Updates random-walk construction to propagate errors (?).
examples/examples_simulation/src/bin/random_walk.rs	Updates random-walk construction to propagate errors (?).
examples/examples_simulation/src/bin/position_simulator.rs	Updates simulator construction to propagate errors (?).
examples/examples_simulation/src/bin/long_call_strategy_simulation.rs	Updates simulator construction to propagate errors (?).
examples/examples_simulation/src/bin/historical_build_chain.rs	Updates random-walk construction to propagate errors (?).

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[joaquinbejar]

Thanks for the review. The single inline finding is addressed in 6c01261.