feat(loan_manager): adjust total_outstanding on liquidation

[EbukaMoses]

close #15

Summary

• Decrement TotalOutstanding when a loan is liquidated, matching repay and check_default
• Add test_liquidation_frees_outstanding_for_approve_loan to verify freed liquidity in approve_loan
• Add .gitignore to exclude target/ build artifacts

Test plan

• cargo fmt --check
• cargo test (all 232 tests pass)

[ogazboiz]

correct accounting fix, merging once ci is green. total_outstanding tracks the sum of original loan principals, added in full on approval and removed in full on every other terminal state (repaid, defaulted single + batch), and liquidation was the one terminal path missing the decrement, permanently inflating outstanding and starving available_liquidity. you add adjust_total_outstanding(&env, &token, -loan.amount) in the effects before the cross-contract calls, consistent with the other three, with checked arithmetic. the test proving a second loan is blocked before liquidation and approvable after is a good one. nice.

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