fix(features): correct debit-spread max profit, width - debit (#16)

[bradsmithmba]

Summary

Position.calculate_max_profit() (src/features/position_models.py) computed the max profit of a BULL_CALL_SPREAD / BEAR_PUT_SPREAD as:

return max(spread_width * abs(self.quantities[0]) - abs(net_credit), abs(net_credit))

For a debit spread, max profit is always spread_width - net_debit. The max(..., debit) fallback returns the debit (the position's max loss) whenever the debit exceeds half the spread width — a value that can even exceed the spread width and is never a valid max profit.

Closes #16.

Fix

spread_width = abs(self.strikes[1] - self.strikes[0])
net_debit = abs(sum(qty * price for qty, price in zip(self.quantities, self.entry_prices)))
return spread_width * abs(self.quantities[0]) - net_debit

Verification

Exercised through the real calculate_max_profit():

Case	width	net debit	old result	new result	correct
debit > half width	1000c	800c	800c (= max loss)	200c	200c
debit < half width	1000c	350c	650c	650c	650c

A regression test (test_debit_spread_max_profit_when_debit_exceeds_half_width) is added to tests/test_position_state.py using the 800c-debit case.

CI note

tests/test_position_state.py imports src.features.position_state, which transitively imports the data layer that is missing until #1 (PR #19) merges, so this test file cannot be collected on master yet. The fix itself was verified by invoking Position.calculate_max_profit() directly (the method has no such dependency). The new test will run in CI once #1 lands.

🤖 Generated with Claude Code

[bradsmithmba]

Merge-ordering note: the regression test added here lives in tests/test_position_state.py, which imports src.features.position_state → the data layer that is missing until #1 (PR #19) merges. On master today this test file fails at collection for that reason, not because of this change. Recommend merging #19 first; after that the new test runs and passes. The fix itself was verified by calling Position.calculate_max_profit() directly (see PR description).