refactor: Remove legacy InternalBalance struct, align PositionView with receipt pattern

- Remove InternalBalance struct and its recordDeposit/recordWithdrawal methods
- Refactor PositionView to use separate creditBalances/debitBalances dictionaries
- Update healthFactor() pure helper to iterate over separate balance dictionaries
- Update maxWithdraw() to accept withdrawType instead of InternalBalance
- Update buildPositionView() to populate new structure directly from receipts
- Update quoteLiquidation() to use new PositionView structure
- Update phase0_pure_math_test.cdc to use new PositionView signature

All 45 tests passing.

Author: kgrgpg