fix(fees): P→E fee dust fixes

[alistair-singh]

1. Stretch gasPad quadratic decay to k=2 — pad stays ~8% at tip==rawGas
2. Use static slippage pad for DOT→ETH swap sizing — removes tip-scaled slippage collapse
3. Add AH DOT ED (0.01 DOT) unconditionally to PayFees estimation — prevents sub-ED refund traps
4. Subtract tip slippage budget from XCM want/remoteFees — relayer absorbs tip-portion drift, raises pool tolerance to ~30%

Addresses Snowfork/snowbridge-app#255