fix(strategies): guard Positive boundaries in butterfly/spread P&L (#387)

[joaquinbejar]

Summary

Six example binaries panicked mid-optimizer-scan with `Positive invariant broken` because strategy P&L and break-even expressions crossed the `Positive` boundary with an unchecked `Add` / `Sub`.

Root-cause fixes:

• `CallButterfly::update_break_even_points`: `strike ± profit/qty` lowered to `Decimal` then wrapped via `Positive::new_decimal(..)`; out-of-range candidates are silently dropped instead of panicking. The same optimizer then skips that combination cleanly.
• `CallButterfly::get_profit_area`: `be[1] - be[0]` and `short_high_strike - short_low_strike` lowered to `Decimal` first, then wrapped; non-positive widths fall back to `Positive::ZERO`.
• `LongButterflySpread::update_break_even_points`: same `Positive ± Decimal` guard.
• `BullPutSpread::get_max_loss`: width computed as `Decimal`, wrapped with `Positive::new_decimal(..).map_err` instead of `Positive - Positive`; inverted-strike candidates now return a typed `MaxLossError` instead of panicking.

Unblocks: `strategy_call_butterfly_best_{area,ratio}`, `strategy_long_butterfly_spread_best_{area,ratio}`, `strategy_call_butterfly_delta`, `strategy_bull_put_spread_extended_delta`.

Closes #387.

Test plan

• All six example binaries run to completion.
• `cargo test --lib --all-features` — 3760 pass, 0 fail.
• `cargo clippy --all-targets --all-features -- -D warnings` clean.
• `cargo fmt --all --check`.

[codecov]

Codecov Report

❌ Patch coverage is 89.47368% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/strategies/bull_put_spread.rs	66.66%	4 Missing ⚠️

Files with missing lines	Coverage Δ
src/strategies/call_butterfly.rs	76.31% <100.00%> (+0.29%)	⬆️
src/strategies/long_butterfly_spread.rs	73.56% <100.00%> (+0.09%)	⬆️
src/strategies/bull_put_spread.rs	76.97% <66.66%> (-0.20%)	⬇️

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.