fix: direction-aware funding projection for all venues

[Flotapponnier]

Summary

• Gains funding rates now stored signed (positive = longs pay, negative = shorts pay)
• computeHlFunding uses pos.isLong — shorts receive HL funding when rate > 0, only pay when rate < 0
• estimateGainsFundingFees applies Math.max(0, rate) for longs, Math.max(0, -rate) for shorts
• estimateCarryFees (dYdX, Paradex, generic) same direction-aware logic
• fetchDydxCarryRates removes Math.abs so negative rates (shorts pay) are preserved

Test plan

• Wallet with known short position on HL: projected Gains funding should be near 0 (or received) when BTC funding rate is positive
• Wallet with long position: projected funding should be positive as before
• dYdX carry path: shorts projected as paying when nextFundingRate < 0, longs when > 0