feat: Implement Chooser Option Pricing Model

[joaquinbejar]

Summary

This PR implements Chooser option pricing (also called as-you-like-it options), where the holder can choose at a specified date whether the option becomes a call or a put.

What Was Implemented

Simple Chooser (Rubinstein 1991)

At the choice date t, the holder chooses max(Call value, Put value).

Formula:

V = S*e^(-qT)*N(d1) - K*e^(-rT)*N(d2) + K*e^(-rt)*N(-y2) - S*e^(-qt)*N(-y1)

Where:

• T = time to final expiration
• t = time to choice date (t < T)
• d1, d2 = standard Black-Scholes d-values for T
• y1 = [ln(S/K) + (b + σ²/2)*t] / (σ√t)
• y2 = y1 - σ√t

Edge Cases

• Choice at expiry (t ≥ T): Returns straddle value (call + put)
• Zero time: Returns max(call intrinsic, put intrinsic)

Integration

• New module: src/pricing/chooser.rs
• Exported via src/pricing/mod.rs
• Routed OptionType::Chooser in black_scholes_model.rs

Why It Was Implemented

Chooser options are important because they:

1. Provide flexibility to decide call/put based on market movement
2. Are valuable in uncertain/volatile markets
3. Are more expensive than vanilla options (due to choice flexibility)
4. Have unique payoff structure: always max(call, put)

Technical Decisions

1. Rubinstein Formula: Closed-form solution for efficiency
2. Choice Date in Days: Stored as days, converted to years for calculations
3. Decimal Arithmetic: Used .to_dec() to avoid Positive type panics on negative intrinsic
4. Straddle at Expiry: When choice_date ≥ expiration, returns call + put

Testing

Added 11 comprehensive unit tests:

• test_simple_chooser: Basic pricing
• test_chooser_more_valuable_than_call / test_chooser_more_valuable_than_put: Fundamental properties
• test_early_choice_date / test_late_choice_date: Edge timing
• test_choice_at_expiry: Straddle case
• test_short_chooser_option: Position negation
• test_zero_time_to_expiry: ATM at expiry = 0
• test_itm_call_at_expiry / test_itm_put_at_expiry: Intrinsic values
• test_higher_vol_means_higher_chooser_value: Volatility property

Verification

• All 3753+ existing tests pass
• make lint-fix pre-push completes successfully
• No Clippy warnings

Closes #242

[codecov]

Codecov Report

❌ Patch coverage is 87.05882% with 11 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/pricing/chooser.rs	88.09%	10 Missing ⚠️
src/pricing/black_scholes_model.rs	0.00%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/pricing/black_scholes_model.rs	54.41% <0.00%> (+1.55%)	⬆️
src/pricing/chooser.rs	88.09% <88.09%> (ø)

... and 1 file with indirect coverage changes

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