ExecutionResult.executed_quantity is always a positive magnitude, so the
old `is_buy = executed_quantity > ZERO` check was always true — every
fill was treated as a buy regardless of order side.
Add an explicit `pub side: OrderSide` field to ExecutionResult and use
`execution.side == OrderSide::Buy` in PositionManager. All construction
sites (source, tests, benchmarks) updated; sell-side tests now use
positive quantities with `side: OrderSide::Sell` instead of the former
negative-quantity hack.
Addresses audit item H4 (position direction always buy).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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